# Changelog

All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
and this project adheres to
[Semantic Versioning](https://semver.org/spec/v2.0.0.html).

## [1.17.0]

### Added

- [#5891](https://github.com/firecracker-microvm/firecracker/pull/5891): Added
  support for virtio device reset.
- [#5983](https://github.com/firecracker-microvm/firecracker/pull/5983): Add two
  optional metrics fields, set via `PUT /metrics` or a config file: `emit_id`
  emits the microVM instance id under a top-level `id` field, and `properties`
  emits operator-defined key-value pairs under a top-level `properties` field.
  Each is opt-in and independent. See [metrics documentation](docs/metrics.md).
- [#6003](https://github.com/firecracker-microvm/firecracker/pull/6003): Added a
  new option `Transparent` for the `huge_pages` setting. If set, Firecracker
  will use transparent huge pages for the guest memory via
  `madvise(MADV_HUGEPAGE)`. Guest memory must be a multiple of 2MB when using
  this option.
- [#6013](https://github.com/firecracker-microvm/firecracker/pull/6013),
  [#6017](https://github.com/firecracker-microvm/firecracker/pull/6017),
  [#6021](https://github.com/firecracker-microvm/firecracker/pull/6021): Added
  official support for 6.18 microVM guest kernels.
- [#6037](https://github.com/firecracker-microvm/firecracker/pull/6037): Added
  support for booting `bzImage` guest kernels on x86_64, in addition to the
  existing uncompressed ELF (`vmlinux`) images. The kernel image format is
  detected automatically, so no configuration change is required.
- [#6064](https://github.com/firecracker-microvm/firecracker/pull/6064): Added
  the `huge_pages` field to `PUT /snapshot/load`, allowing the restored microVM
  to reuse the snapshot's host page configuration or select `None`,
  `Transparent`, or `2M`.
- [#6109](https://github.com/firecracker-microvm/firecracker/pull/6109): Added
  an optional `sync_snapshot_files` field to the `PUT /snapshot/create` API,
  defaulting to `true`. It controls whether the snapshot state and guest memory
  files are synced to disk (`fsync`) before the request returns. Setting it to
  `false` returns without waiting for the writeback, making snapshot creation
  faster at the cost of durability across a host crash; the data stays in the
  host page cache, so same-host reads still see the full contents. Block device
  backing files are always `fsync`'d regardless. See
  [snapshot documentation](docs/snapshotting/snapshot-support.md).
- [#6116](https://github.com/firecracker-microvm/firecracker/pull/6116): On
  aarch64, enable `KVM_CAP_ARM_WRITABLE_IMP_ID_REGS` when the host kernel offers
  it (Linux 6.15 and later), adding support for custom CPU templates that modify
  the implementation ID registers (`MIDR_EL1`, `REVIDR_EL1`, `AIDR_EL1`). Such
  templates previously failed at boot with
  `Failed to set register ... Invalid argument` because KVM rejects the write
  unless the capability is enabled on the VM.
- [#6145](https://github.com/firecracker-microvm/firecracker/pull/6145): Added
  official support for AWS Graviton5 (m9g.metal-48xl) instances.
- [#6098](https://github.com/firecracker-microvm/firecracker/pull/6098): Added
  new VIRTIO_BLK_F_BLK_SIZE and VIRTIO_BLK_F_TOPOLOGY features to the
  virtio-block device. More information is in the new [block](docs/block.md)
  documentation.
- [#6142](https://github.com/firecracker-microvm/firecracker/pull/6142): Add
  opt-in virtio-blk discard support for writable `Sync` IO engine drives through
  the `discard` drive configuration field. See the
  [block discard documentation](docs/api_requests/block-discard.md).

### Changed

- [#6115](https://github.com/firecracker-microvm/firecracker/pull/6115): Changed
  the T2S CPU template to set the `FB_CLEAR` bit of `MSR_IA32_ARCH_CAPABILITIES`
  to 1. This lets guest kernels recognize that the `VERW` instruction clears
  fill buffers, including on host kernels before v6.4 that cannot expose
  `FLUSH_L1D`.
- [#6172](https://github.com/firecracker-microvm/firecracker/pull/6172): Gate
  CLIRD_EL1 override to only happen on host kernels equal or newer than 6.10
  release. This aliviates the guest performance regression seen on host kernels
  in range \[6.3..6.10) correlated to incorrect cpu cache topology presented to
  the guest.

### Deprecated

### Removed

### Fixed

- [#6100](https://github.com/firecracker-microvm/firecracker/pull/6100): Fixed
  the vsock device permanently suppressing RX (host-to-guest) delivery after a
  bare pause/resume cycle (`PATCH /vm` with `Paused` then `Resumed`, without a
  snapshot). The resume kick armed the `TRANSPORT_RESET` RX gate even though no
  reset event had been sent, so the guest could never acknowledge it and every
  new host-initiated connection made after the resume hung forever. The gate is
  now part of the persisted device state and the resume kick respects it instead
  of arming it.
- [#6174](https://github.com/firecracker-microvm/firecracker/pull/6174): Fixed
  `virtio-mem` leaving unplugged memory writable by the VMM, and potentially
  unmapped, on microVMs restored from a snapshot memory file. Firecracker now
  maps each slot straight to the protection it should have, with the
  `MAP_NORESERVE` flag, and aborts if the re-map fails.
- [#6174](https://github.com/firecracker-microvm/firecracker/pull/6174): Fixed
  `virtio-mem` discarding the whole hotpluggable region on every `UNPLUG_ALL`
  request, even when nothing was plugged. The discard is now skipped when the
  range has no plugged blocks.
- [#6176](https://github.com/firecracker-microvm/firecracker/pull/6176): Fixed
  `virtio-mem` leaving its block accounting inconsistent with the KVM memory
  slots if a plug or unplug request failed part way through. Firecracker now
  commits each slot's state only after its KVM update succeeds, so a partial
  failure leaves the block state and the KVM slots reflecting exactly the slots
  that were updated.
- [#5956](https://github.com/firecracker-microvm/firecracker/pull/5956): Fixed a
  TOCTOU race in the aarch64 jailer when setting ownership of the CPU cache and
  `MIDR_EL1` information files copied into the chroot.
- [#6076](https://github.com/firecracker-microvm/firecracker/pull/6076): Fixed
  memory hotplug sizes silently wrapping when converted from MiB to bytes.
  `requested_size_mib` on `PATCH /hotplug/memory` and `total_size_mib`,
  `block_size_mib` and `slot_size_mib` on `PUT /hotplug/memory` are now bounded
  to 32 bits. Values above that previously wrapped to a smaller byte count, so a
  large enough request was accepted as a 0-byte region instead of being
  rejected.
- [#6031](https://github.com/firecracker-microvm/firecracker/pull/6031),
  [#6041](https://github.com/firecracker-microvm/firecracker/pull/6041),
  [#6077](https://github.com/firecracker-microvm/firecracker/pull/6077): Fixed
  the vsock device re-arming its host-stream `EPOLLIN` interest while received
  data was still awaiting a guest RX buffer, which could busy-spin the event
  thread until the guest posted buffers. The device now drains the host stream
  fully across successive RX operations instead of one packet per event loop
  iteration, reducing the host-side CPU cost per gigabit of host-to-guest
  traffic by up to ~50% and improving host-to-guest throughput by ~44% (median)
  in most configurations. On single-vcpu microVMs on the newest Intel hosts
  (m7i, m8i), host-to-guest throughput may instead decrease by ~15% (median),
  where the single guest vCPU rather than the host becomes the bottleneck.
  Terminating a connection now also discards its TX buffer, so the device stops
  advertising `EPOLLOUT` for a host stream it will never write to again, which
  could otherwise busy-spin the event thread indefinitely.
- [#6086](https://github.com/firecracker-microvm/firecracker/pull/6086),
  [#6143](https://github.com/firecracker-microvm/firecracker/pull/6143): Fixed a
  deadlock in the logger: a signal handler that logs while the interrupted
  thread is already logging would hang the VMM and its API socket. This is
  reachable whenever a `SIGPIPE` is raised by Firecracker's own log write, for
  example when logging to `stdout` and the reader of that pipe exits. The logger
  now uses an `RwLock`, and `sigpipe_handler` only increments the
  `signals.sigpipe` metric instead of logging, so it no longer emits a
  `Received signal 13, code 0.` line.
- [#6120](https://github.com/firecracker-microvm/firecracker/pull/6120): Fixed a
  bug caused by a KVM behavior change introduced in Linux 6.13, which requires
  guest CPUID to be set before userspace reads CPUID-dependent MSRs. On x86_64
  with Linux 6.18 host kernels, Firecracker read the base values of those MSRs
  before setting guest CPUID, so KVM returned zero for such MSRs and CPU
  templates consequently used zero for bits configured for passthrough.
  Firecracker now sets guest CPUID before reading MSRs to be modified by CPU
  templates, so passthrough bits retain their KVM-provided values and features
  such as eIBRS remain exposed to the guest.

## [1.16.1]

### Fixed

- [#5959](https://github.com/firecracker-microvm/firecracker/pull/5959):
  Reverted the use of `O_NOFOLLOW` for the jailer's cgroup and network namespace
  file operations, so symlinks are again allowed in these paths.
- [#5958](https://github.com/firecracker-microvm/firecracker/pull/5958): Fixed a
  bug that caused vsock guest-to-host connections to time out after snapshot
  restore, triggered by taking a snapshot with a TX descriptor in-flight. On
  restore the device now replays the TX queue notification so in-flight TX
  descriptors are re-processed and notification suppression is re-armed.

## [1.16.0]

### Added

- [#5786](https://github.com/firecracker-microvm/firecracker/pull/5786): Added
  developer preview support for hotplugging and hot-unplugging PCI virtio
  devices (block, pmem, net) on a running microVM. The guest must manually
  rescan the PCI bus after hotplug and remove the device before unplug since no
  automatic notification mechanism is implemented yet. More information can be
  found in the [Device Hotplugging](docs/device-hotplug.md) documentation page.
- [#5323](https://github.com/firecracker-microvm/firecracker/pull/5323): Add
  support for Vsock Unix domain socket path overriding on snapshot restore. More
  information can be found in the
  [docs](docs/vsock.md/#unix-domain-socket-renaming).
- [#5824](https://github.com/firecracker-microvm/firecracker/pull/5824): Add
  optional rate limiting to serial console output, configurable via the
  `rate_limiter` field on `PUT /serial`. A new metric is exposed under `uart`:
  `rate_limiter_dropped_bytes`.
- [#5799](https://github.com/firecracker-microvm/firecracker/pull/5799): Add
  per-callsite rate limiting for error, warn, and info level log messages. Each
  callsite independently allows up to 10 messages per 5-second window. When
  logging resumes after suppression, a warn-level summary reports the count of
  suppressed messages. A new `rate_limited_log_count` metric tracks the total
  number of suppressed messages.
- [#5789](https://github.com/firecracker-microvm/firecracker/pull/5789): Add
  rate-limiter support to virtio-pmem device to allow control over I/O bandwidth
  generated by the FLUSH requests from the guest.
- [#5872](https://github.com/firecracker-microvm/firecracker/pull/5872): Add
  notification suppression support in the virtio-vsock device via the EVENT_IDX
  virtio feature to reduce device overhead.
- [#5828](https://github.com/firecracker-microvm/firecracker/pull/5828):
  Advertise MTU to the guest via `VIRTIO_NET_F_MTU` using a new optional `mtu`
  field in the `network-interfaces` API. When set, a compatible guest driver
  will configure the interface with the specified MTU.
- [#5906](https://github.com/firecracker-microvm/firecracker/pull/5906): Add
  `rng-seed` FDT node for aarch64 guests which provides an initial random seed
  for the guest to use. This helps older aarch64 machines which do not have
  hardware random generators.
- Added support for Linux 6.18 host kernels alongside the existing 5.10 and 6.1
  host kernels. See the [kernel support policy](docs/kernel-policy.md) for
  details.

### Changed

### Deprecated

### Removed

### Fixed

- [#5762](https://github.com/firecracker-microvm/firecracker/pull/5762): Cap
  virtio-rng per-request entropy to 64 KiB. Previously, a guest could construct
  a descriptor chain that caused Firecracker to allocate more host memory than
  the guest actually provided, potentially leading to excessive host memory
  consumption.
- [#5760](https://github.com/firecracker-microvm/firecracker/pull/5760): Fixed
  HID (Hardware ID) of VMGenID device so that it aligns with the upstream Linux
  kernel. This caused the driver not to be bound correctly to the device prior
  to Linux kernel 6.10.
- [#5764](https://github.com/firecracker-microvm/firecracker/pull/5764): Fixed a
  bug that caused the guest UART driver to get stuck and stop transmitting after
  snapshot restore. The bug was triggered by taking a snapshot while a serial
  transmission was taking place. On restore the driver would wait for a TX
  interrupt that would never arrive and no output would appear in the serial
  console.
- [#5780](https://github.com/firecracker-microvm/firecracker/pull/5780): Fixed
  missing `/sys/devices/system/cpu/cpu*/cache/*` in aarch64 guests when running
  on host kernels >= 6.3 with guest kernels >= 6.1.156.
- [#5793](https://github.com/firecracker-microvm/firecracker/pull/5793): Fixed
  virtio-mem plug/unplug skipping KVM slot updates for memory blocks not aligned
  to a slot boundary. On plug, this could leave hotplugged memory inaccessible
  to the guest. On unplug, the guest could retain access to memory that
  Firecracker considered freed.
- [#5794](https://github.com/firecracker-microvm/firecracker/pull/5794): Bound
  balloon statistics descriptor length to prevent a guest-controlled oversized
  descriptor from temporarily stalling the VMM event loop. Only affects microVMs
  with `stats_polling_interval_s > 0`.
- [#5809](https://github.com/firecracker-microvm/firecracker/pull/5809): Fixed a
  bug on host Linux >= 5.16 for x86_64 guests using the `kvm-clock` clock source
  causing the monotonic clock to jump on restore by the wall-clock time elapsed
  since the snapshot was taken. Users using `kvm-clock` that want to explicitly
  advance the clock with `KVM_CLOCK_REALTIME` can opt back in using the new
  `clock_realtime` flag in `LoadSnapshot` API.
- [#5738](https://github.com/firecracker-microvm/firecracker/pull/5738): Fixed
  x86_64 snapshot serialization to cover the full KVM custom MSR range
  (0x4b564d00-0x4b564dff) instead of a small subset. Previously, some KVM MSRs
  such as MSR_KVM_ASYNC_PF_INT and MSR_KVM_ASYNC_PF_ACK were missing from
  snapshots, which could cause issues on restore.
- [#5818](https://github.com/firecracker-microvm/firecracker/pull/5818): Enforce
  the virtio device initialization sequence in the PCI transport, matching the
  existing MMIO transport behavior. The PCI transport now validates device
  status transitions, rejects queue configuration writes outside the FEATURES_OK
  to DRIVER_OK window, rejects feature negotiation outside the DRIVER state,
  blocks re-initialization after a failed reset, and sets DEVICE_NEEDS_RESET
  when device activation fails.
- [#5818](https://github.com/firecracker-microvm/firecracker/pull/5818): Reject
  device status writes that clear previously set bits in the MMIO transport,
  except for reset.
- [#5884](https://github.com/firecracker-microvm/firecracker/pull/5884):
  Corrected the OpenAPI spec for `PATCH /balloon/hinting/start` and
  `PATCH /balloon/hinting/stop` to declare `204 No Content` instead of `200`,
  matching the actual runtime response.
- [#5882](https://github.com/firecracker-microvm/firecracker/pull/5882): Fixed a
  race in the vsock device where, after snapshot restore, the RX queue could
  deliver data to the guest before it had acknowledged the TRANSPORT_RESET
  event, causing established connections to break.

## [1.15.0]

### Added

- [#5510](https://github.com/firecracker-microvm/firecracker/pull/5510),
  [#5593](https://github.com/firecracker-microvm/firecracker/pull/5593),
  [#5564](https://github.com/firecracker-microvm/firecracker/pull/5564): Add
  support for the
  [VMClock device](https://uapi-group.org/specifications/specs/vmclock). The
  implementation supports the snapshot safety features proposed
  [here](https://lore.kernel.org/lkml/20260107132514.437-1-bchalios@amazon.es/),
  but doesn't provide currently any clock-specific information for helping the
  guest synchronize its clocks. More information can be found in
  [docs](docs/snapshotting/snapshot-support.md#userspace-notifications-of-loading-virtual-machine-snapshots).

- [#5574](https://github.com/firecracker-microvm/firecracker/pull/5574),
  [#5671](https://github.com/firecracker-microvm/firecracker/pull/5671),
  [#5674](https://github.com/firecracker-microvm/firecracker/pull/5674)
  [#5690](https://github.com/firecracker-microvm/firecracker/pull/5690) Added
  Intel Granite Rapids as a supported and tested platform for Firecracker on 6.1
  host kernel versions.

### Changed

- [#5564](https://github.com/firecracker-microvm/firecracker/pull/5564): which
  added support for VMClock, uses one extra GSI for the VMClock device itself
  which reduces the available GSIs for VirtIO devices. New maximum values is 92
  devices on Aarch64 and 17 devices on x86.
- [#5631](https://github.com/firecracker-microvm/firecracker/pull/5631): Update
  binary copy process inside Jailer to disallow symlinks and hardlinks at the
  destination path and change ownership of the copied binary to the specified
  uid/gid.

### Deprecated

### Removed

### Fixed

- [#5688](https://github.com/firecracker-microvm/firecracker/pull/5688): Fixed
  vsock local port reuse across snapshot restore by saving the last used local
  port into the snapshot, so users need to regenerate snapshots.
- [#5698](https://github.com/firecracker-microvm/firecracker/pull/5698): Fixed
  the possible ENXIO error which could occur during file open operation if the
  underlying file is FIFO without active readers already attached.
- [#5705](https://github.com/firecracker-microvm/firecracker/pull/5705): Fixed a
  bug that caused Firecracker to corrupt the memory files of differential
  snapshots for VMs with multiple memory slots. This affected VMs using memory
  hot-plugging or any x86 VMs with a memory size larger than 3GiB.
- [#5739](https://github.com/firecracker-microvm/firecracker/pull/5739): Fixed
  validation of TCP SYN options length when MMDS is enabled.

## [1.14.0]

### Added

- [#5350](https://github.com/firecracker-microvm/firecracker/pull/5350): Added a
  `/serial` endpoint, which allows setting `serial_out_path` to the path of a
  pre-created file into which Firecracker should redirect output from the
  guest's serial console. Not configuring it means Firecracker will continue to
  print serial output to stdout. Similarly to the logger, this configuration is
  not persisted across snapshots.
- [#5463](https://github.com/firecracker-microvm/firecracker/pull/5463): Added
  support for `virtio-pmem` devices. See [documentation](docs/pmem.md) for more
  information.
- [#5534](https://github.com/firecracker-microvm/firecracker/pull/5534): Added
  support for memory hot-plugging through the `virtio-mem` device. See
  [documentation](docs/memory-hotplug.md) for more information.
- [#5491](https://github.com/firecracker-microvm/firecracker/pull/5491): Added
  support for `virtio-balloon` free page reporting and hinting. Free page
  reporting is a developer preview not for production feature. See
  [documentation](docs/ballooning.md) for more information.

### Changed

- [#4028](https://github.com/firecracker-microvm/firecracker/pull/4028):
  Firecracker now creates the log and metrics files if they do not exist,
  simplifying the launch of Firecracker by removing a manual step.
- [#5516](https://github.com/firecracker-microvm/firecracker/pull/5516): Balloon
  stats now supports guest kernel >= 6.12, adding metrics on guest OOM kills,
  memory allocation stalls, and memory scan/reclaim info.
- [#5526](https://github.com/firecracker-microvm/firecracker/pull/5526): Specify
  IA32_MTRRdefType MSR on VM boot to allow it to set page attributes for memory
  regions.

### Deprecated

### Removed

- [#5439](https://github.com/firecracker-microvm/firecracker/pull/5439): Removed
  the `rx_partial_writes`, `tx_partial_reads`, `sync_response_fails`,
  `sync_vmm_send_timeout_count`, `deprecated_cmd_line_api_calls`, `log_fails`
  and `device_events` metrics, as they were never incremented.

### Fixed

- [#5418](https://github.com/firecracker-microvm/firecracker/pull/5418): Fixed
  typo in Swagger definition of `MmdsConfig`, where the property `imds_compat`
  was spelled as `imds_comat`. This caused auto-generated clients to create bad
  requests.
- [#5447](https://github.com/firecracker-microvm/firecracker/pull/5447): Fixed
  Intel AMX enabling for kernels that support dynamic XSTATE features for
  userspace applications but not for KVM guests (e.g. kernel versions >= 5.16
  and < 5.17).
- [#5485](https://github.com/firecracker-microvm/firecracker/pull/5485): Fixed a
  bug causing a read/write from an iovec to be duplicated when receiving an
  error on an iovec other than the first. This caused a data corruption issue in
  the vsock device starting from guest kernel 6.17.
- [#5494](https://github.com/firecracker-microvm/firecracker/pull/5494): Fixed a
  watchdog soft lockup bug on microVMs restored from snapshots by calling
  KVM_KVMCLOCK_CTRL ioctl before resuming.
- [#5538](https://github.com/firecracker-microvm/firecracker/pull/5538): Fixed a
  cache coherency issue on non-FWB aarch64 platforms by adding `dma-coherent`
  property to virtio-mmio nodes in the FDT.

## [1.13.0]

### Added

- [#5139](https://github.com/firecracker-microvm/firecracker/pull/5139): Added
  support for [PVTime](https://docs.kernel.org/virt/kvm/arm/pvtime.html). This
  is used to support steal time on ARM machines.
- [#5175](https://github.com/firecracker-microvm/firecracker/pull/5175): Allow
  including a custom cpu template directly in the json configuration file passed
  to `--config-file` under the `cpu_config` key.
- [#5274](https://github.com/firecracker-microvm/firecracker/pull/5274): Allow
  taking diff snapshots even if dirty page tracking is disabled, by using
  `mincore(2)` to overapproximate the set of dirty pages. Only works if swap is
  disabled.
- [#5290](https://github.com/firecracker-microvm/firecracker/pull/5290):
  Extended MMDS to support the EC2 IMDS-compatible session token headers (i.e.
  "X-aws-ec2-metadata-token" and "X-aws-ec2-metadata-token-ttl-seconds")
  alongside the MMDS-specific ones.
- [#5290](https://github.com/firecracker-microvm/firecracker/pull/5290): Added
  `mmds.rx_invalid_token` and `mmds.rx_no_token` metrics to track the number of
  GET requests that were rejected due to token validation failures in MMDS
  version 2. These metrics also count requests that would be rejected in MMDS
  version 2 when MMDS version 1 is configured. They helps users assess readiness
  for migrating to MMDS version 2.
- [#5310](https://github.com/firecracker-microvm/firecracker/pull/5310): Added
  an optional `imds_compat` field (default to false if not provided) to PUT
  requests to `/mmds/config` to enforce MMDS to always respond plain text
  contents in the IMDS format regardless of the `Accept` header in requests.
  Users need to regenerate snapshots.
- [#5364](https://github.com/firecracker-microvm/firecracker/pull/5364): Added
  PCI support in Firecracker. PCI support is optional. Users can enable it
  passing the `--enable-pci` flag when launching the Firecracker process. When
  Firecracker process is launched with PCI support, it will create all VirtIO
  devices using a PCI VirtIO transport. If not enabled, Firecracker will use the
  MMIO transport instead.

### Changed

- [#5165](https://github.com/firecracker-microvm/firecracker/pull/5165): Changed
  Firecracker snapshot feature from developer preview to generally available.
  Incremental snapshots remain in developer preview.
- [#5282](https://github.com/firecracker-microvm/firecracker/pull/5282): Updated
  jailer to no longer require the executable file name to contain `firecracker`.
- [#5290](https://github.com/firecracker-microvm/firecracker/pull/5290): Changed
  MMDS to validate the value of "X-metadata-token-ttl-seconds" header only if it
  is a PUT request to /latest/api/token, as in EC2 IMDS.
- [#5290](https://github.com/firecracker-microvm/firecracker/pull/5290): Changed
  MMDS version 1 to support the session oriented method as in version 2,
  allowing easier migration to version 2. Note that MMDS version 1 accepts a GET
  request even with no token or an invalid token so that existing workloads
  continue to work.

### Deprecated

- [#5274](https://github.com/firecracker-microvm/firecracker/pull/5274):
  Deprecated the `enable_diff_snapshots` parameter of the `/snapshot/load` API.
  Use `track_dirty_pages` instead.

### Removed

- [#5411](https://github.com/firecracker-microvm/firecracker/pull/5411): Removed
  official support for Intel Skylake instances. Firecracker will continue to
  work on those instances, but we will no longer perform automated testing on
  them.

### Fixed

- [#5222](https://github.com/firecracker-microvm/firecracker/pull/5222): Fixed
  network and rng devices locking up on hosts with non 4K pages.
- [#5226](https://github.com/firecracker-microvm/firecracker/pull/5226): Fixed
  MMDS to set `Content-Type` header correctly (i.e. `Content-Type: text/plain`
  for IMDS-formatted or error responses and `Content-Type: application/json` for
  JSON-formatted responses).
- [#5260](https://github.com/firecracker-microvm/firecracker/pull/5260): Fixed a
  bug allowing the block device to starve all other devices when backed by a
  sufficiently slow drive.
- [#4207](https://github.com/firecracker-microvm/firecracker/issues/4207): Fixed
  GSI numbering on aarch64 to correctly allow up to 96 devices being attached
  simultaneously.
- [#5290](https://github.com/firecracker-microvm/firecracker/pull/5290): Fixed
  MMDS to reject PUT requests containing `X-Forwarded-For` header regardless of
  its casing (e.g. `x-forwarded-for`).
- [#5328](https://github.com/firecracker-microvm/firecracker/pull/5328): Fixed
  MMDS to set the token TTL header (i.e. "X-metadata-token-ttl-seconds" or
  "X-aws-ec2-metadata-token-ttl-seconds") in the response to "PUT
  /latest/api/token", as EC2 IMDS does.

## [1.12.0]

### Added

- [#5048](https://github.com/firecracker-microvm/firecracker/pull/5048): Added
  support for [PVH boot mode](docs/pvh.md). This is used when an x86 kernel
  provides the appropriate ELF Note to indicate that PVH boot mode is supported.
  Linux kernels newer than 5.0 compiled with `CONFIG_PVH=y` set this ELF Note,
  as do FreeBSD kernels.
- [#5065](https://github.com/firecracker-microvm/firecracker/pull/5065) Added
  support for Intel AMX (Advanced Matrix Extensions). To be able to take and
  restore a snapshot of Intel AMX state, `Xsave` is used instead of `kvm_xsave`,
  so users need to regenerate snapshots.
- [#4731](https://github.com/firecracker-microvm/firecracker/pull/4731): Added
  support for modifying the host TAP device name during snapshot restore.
- [#5146](https://github.com/firecracker-microvm/firecracker/pull/5146): Added
  Intel Sapphire Rapids as a supported and tested platform for Firecracker.
- [#5148](https://github.com/firecracker-microvm/firecracker/pull/5148): Added
  ARM Graviton4 as a supported and tested platform for Firecracker.

### Changed

- [#5118](https://github.com/firecracker-microvm/firecracker/pull/5118): Cleared
  WAITPKG CPUID bit in CPUID normalization. The feature enables a guest to put a
  physical processor into an idle state, which is undesirable in a FaaS
  environment since that is what the host wants to decide.
- [#5142](https://github.com/firecracker-microvm/firecracker/pull/5142):
  Clarified what CPU models are supported by each existing CPU template.
  Firecracker exits with an error if a CPU template is used on an unsupported
  CPU model.

### Deprecated

- [#4948](https://github.com/firecracker-microvm/firecracker/pull/4948):
  Deprecated the `page_size_kib` field in the
  [UFFD handshake](docs/snapshotting/handling-page-faults-on-snapshot-resume.md#registering-memory-to-be-handled-via-userfault-file-descriptors),
  and replaced it with a `page_size` field. The `page_size_kib` field is
  misnamed, as the value Firecracker sets it to is actually the page size in
  _bytes_, not KiB. It will be removed in Firecracker 2.0.

### Fixed

- [#5074](https://github.com/firecracker-microvm/firecracker/pull/5074) Fix the
  `SendCtrlAltDel` command not working for ACPI-enabled guest kernels, by
  dropping the i8042.nopnp argument from the default kernel command line
  Firecracker constructs.
- [#5122](https://github.com/firecracker-microvm/firecracker/pull/5122): Keep
  the UFFD Unix domain socket open to prevent the race condition between the
  guest memory mappings message and the shutdown event that was sometimes
  causing arrival of an empty message on the UFFD handler side.
- [#5143](https://github.com/firecracker-microvm/firecracker/pull/5143): Fixed
  to report `process_startup_time_us` and `process_startup_time_cpu_us` metrics
  for `api_server` right after the API server starts, while previously reported
  before applying seccomp filter and starting the API server. Users may observe
  a bit longer startup time metrics.

## [1.11.0]

### Added

- [#4987](https://github.com/firecracker-microvm/firecracker/pull/4987): Reset
  physical counter register (`CNTPCT_EL0`) on VM startup. This avoids VM reading
  the host physical counter value. This is only possible on 6.4 and newer
  kernels. For older kernels physical counter will still be passed to the guest
  unmodified. See more info
  [here](https://github.com/firecracker-microvm/firecracker/blob/main/docs/prod-host-setup.md#arm-only-vm-physical-counter-behaviour)
- [#5088](https://github.com/firecracker-microvm/firecracker/pull/5088): Added
  AMD Genoa as a supported and tested platform for Firecracker.

### Changed

- [#4913](https://github.com/firecracker-microvm/firecracker/pull/4913): Removed
  unnecessary fields (`max_connections` and `max_pending_resets`) from the
  snapshot format, bumping the snapshot version to 5.0.0. Users need to
  regenerate snapshots.
- [#4926](https://github.com/firecracker-microvm/firecracker/pull/4926): Replace
  underlying implementation for seccompiler from in house one in favor of
  `libseccomp` which produces smaller and more optimized BPF code.

### Deprecated

### Removed

### Fixed

- [#4921](https://github.com/firecracker-microvm/firecracker/pull/4921): Fixed
  swagger `CpuConfig` definition to include missing aarch64-specific fields.
- [#4916](https://github.com/firecracker-microvm/firecracker/pull/4916): Fixed
  `IovDeque` implementation to work with any host page size. This fixes
  virtio-net device on non 4K host kernels.
- [#4991](https://github.com/firecracker-microvm/firecracker/pull/4991): Fixed
  `mem_size_mib` and `track_dirty_pages` being mandatory for all
  `PATCH /machine-config` requests. Now, they can be omitted which leaves these
  parts of the machine configuration unchanged.
- [#5007](https://github.com/firecracker-microvm/firecracker/pull/5007): Fixed
  watchdog softlockup warning on x86_64 guests when a vCPU is paused during GDB
  debugging.
- [#5021](https://github.com/firecracker-microvm/firecracker/pull/5021) If a
  balloon device is inflated post UFFD-backed snapshot restore, Firecracker now
  causes `remove` UFFD messages to be sent to the UFFD handler. Previously, no
  such message would be sent.
- [#5034](https://github.com/firecracker-microvm/firecracker/pull/5034): Fix an
  integer underflow in the jailer when computing the value it passes to
  Firecracker's `--parent-cpu-time-us` values, which caused development builds
  of Firecracker to crash (but production builds were unaffected as underflows
  do not panic in release mode).
- [#5045](https://github.com/firecracker-microvm/firecracker/pull/5045): Fixed
  an issue where firecracker intermittently receives SIGHUP when using jailer
  with `--new-pid-ns` but without `--daemonize`.
- [#4995](https://github.com/firecracker-microvm/firecracker/pull/4995):
  Firecracker no longer overwrites CPUID leaf 0x80000000 when running AMD
  hardware, meaning the guest can now discover a greater range of CPUID leaves
  in the extended function range (this range is host kernel dependent).
- [#5046](https://github.com/firecracker-microvm/firecracker/pull/5046): Retry
  KVM_CREATE_VM on EINTR that occasionally happen on heavily loaded hosts to
  improve reliability of microVM creation.
- [#5052](https://github.com/firecracker-microvm/firecracker/pull/5052): Build
  the empty seccomp policy as default for debug builds to avoid crashes on
  syscalls introduced by debug assertions from Rust 1.80.0.

## [1.10.0]

### Added

- [#4834](https://github.com/firecracker-microvm/firecracker/pull/4834): Add
  `VIRTIO_NET_F_RX_MRGBUF` support to the `virtio-net` device. When this feature
  is negotiated, guest `virtio-net` driver can perform more efficient memory
  management which in turn improves RX and TX performance.
- [#4460](https://github.com/firecracker-microvm/firecracker/pull/4460): Add a
  call to
  [`KVM_KVMCLOCK_CTRL`](https://docs.kernel.org/virt/kvm/api.html#kvm-kvmclock-ctrl)
  after pausing vCPUs on x86_64 architectures. This ioctl sets a flag in the KVM
  state of the vCPU indicating that it has been paused by the host userspace. In
  guests that use kvmclock, the soft lockup watchdog checks this flag. If it is
  set, it won't trigger the lockup condition. Calling the ioctl for guests that
  don't use kvmclock will fail. These failures are not fatal. We log the failure
  and increase the `vcpu.kvmclock_ctrl_fails` metric.
- [#4869](https://github.com/firecracker-microvm/firecracker/pull/4869): Added
  support for Aarch64 systems which feature CPU caches with a number of sets
  higher than `u16::MAX`.
- [#4797](https://github.com/firecracker-microvm/firecracker/pull/4797),
  [#4854](https://github.com/firecracker-microvm/firecracker/pull/4854): Added
  GDB debugging support for a microVM guest kernel. Please see our
  [GDB debugging documentation](docs/gdb-debugging.md) for more information.

### Changed

- [#4844](https://github.com/firecracker-microvm/firecracker/pull/4844): Upgrade
  `virtio-net` device to use `readv` syscall to avoid unnecessary memory copies
  on RX path, increasing the RX performance.

### Deprecated

### Removed

- [#4804](https://github.com/firecracker-microvm/firecracker/pull/4804): Drop
  Support for guest kernel 4.14. Linux 4.14 reached end-of-life in
  [January 2024](https://lore.kernel.org/lkml/2024011046-ecology-tiptoeing-ce50@gregkh/)
  The minimum supported guest kernel now is 5.10.

### Fixed

- [#4796](https://github.com/firecracker-microvm/firecracker/pull/4796): Fixed
  Vsock not notifying guest about `TRANSPORT_RESET_EVENT` event after snapshot
  restore. This resulted in guest waiting indefinitely on a connection which was
  reset during snapshot creation.
- [#4790](https://github.com/firecracker-microvm/firecracker/pull/4790): v1.9.0
  was missing most of the debugging information in the debuginfo file, due to a
  change in the Cargo defaults. This has been corrected.
- [#4826](https://github.com/firecracker-microvm/firecracker/pull/4826): Add
  missing configuration of tap offload features when restoring from a snapshot.
  Setting the features was previously
  [moved](https://github.com/firecracker-microvm/firecracker/pull/4680/commits/49ed5ea4b48ccd98903da037368fa3108f58ac1f)
  from net device creation to device activation time, but it was not reflected
  in the restore path. This was leading to inability to connect to the restored
  VM if the offload features were used.

## [1.9.0]

### Added

- [#4687](https://github.com/firecracker-microvm/firecracker/pull/4687): Added
  VMGenID support for microVMs running on ARM hosts with 6.1 guest kernels.
  Support for VMGenID via DeviceTree bindings exists only on mainline 6.10 Linux
  onwards. Users of Firecracker will need to backport the relevant patches on
  top of their 6.1 kernels to make use of the feature. As a result, Firecracker
  snapshot version is now 3.0.0
- [#4732](https://github.com/firecracker-microvm/firecracker/pull/4732),
  [#4733](https://github.com/firecracker-microvm/firecracker/pull/4733),
  [#4741](https://github.com/firecracker-microvm/firecracker/pull/4741),
  [#4746](https://github.com/firecracker-microvm/firecracker/pull/4746): Added
  official support for 6.1 microVM guest kernels.
- [#4743](https://github.com/firecracker-microvm/firecracker/pull/4743): Added
  support for `-h` help flag to the Jailer. The Jailer will now print the help
  message with either `--help` or `-h`.

### Changed

### Deprecated

- Support for guest kernel 4.14 is now deprecated. We will completely remove
  4.14 support with Firecracker version v1.10

### Removed

- [#4689](https://github.com/firecracker-microvm/firecracker/pull/4689): Drop
  support for host kernel 4.14. Linux 4.14 reached end-of-life in
  [January 2024](https://lore.kernel.org/lkml/2024011046-ecology-tiptoeing-ce50@gregkh/).
  The minimum supported kernel now is 5.10. Guest kernel 4.14 is still
  supported.

### Fixed

- [4680](https://github.com/firecracker-microvm/firecracker/pull/4680): Fixed an
  issue
  ([#4659](https://github.com/firecracker-microvm/firecracker/issues/4659))
  where the virtio-net device implementation would always assume the guest
  accepts all VirtIO features the device offers. This is always true with the
  Linux guest kernels we are testing but other kernels, like FreeBSD make
  different assumptions. This PR fixes the emulation code to set the TAP
  features based on the features accepted by the guest.

## [1.8.0]

### Added

- [#4428](https://github.com/firecracker-microvm/firecracker/pull/4428): Added
  ACPI support to Firecracker for x86_64 microVMs. Currently, we pass ACPI
  tables with information about the available vCPUs, interrupt controllers,
  VirtIO and legacy x86 devices to the guest. This allows booting kernels
  without MPTable support. Please see our
  [kernel policy documentation](docs/kernel-policy.md) for more information
  regarding relevant kernel configurations.
- [#4487](https://github.com/firecracker-microvm/firecracker/pull/4487): Added
  support for the Virtual Machine Generation Identifier (VMGenID) device on
  x86_64 platforms. VMGenID is a virtual device that allows VMMs to notify
  guests when they are resumed from a snapshot. Linux includes VMGenID support
  since version 5.18. It uses notifications from the device to reseed its
  internal CSPRNG. Please refer to
  [snapshot support](docs/snapshotting/snapshot-support.md) and
  [random for clones](docs/snapshotting/random-for-clones.md) documention for
  more info on VMGenID. VMGenID state is part of the snapshot format of
  Firecracker. As a result, Firecracker snapshot version is now 2.0.0.

### Changed

- [#4492](https://github.com/firecracker-microvm/firecracker/pull/4492): Changed
  `--config` parameter of `cpu-template-helper` optional. Users no longer need
  to prepare kernel, rootfs and Firecracker configuration files to use
  `cpu-template-helper`.
- [#4537](https://github.com/firecracker-microvm/firecracker/pull/4537) Changed
  T2CL template to pass through bit 27 and 28 of `MSR_IA32_ARCH_CAPABILITIES`
  (`RFDS_NO` and `RFDS_CLEAR`) since KVM consider they are able to be passed
  through and T2CL isn't designed for secure snapshot migration between
  different processors.
- [#4537](https://github.com/firecracker-microvm/firecracker/pull/4537) Changed
  T2S template to set bit 27 of `MSR_IA32_ARCH_CAPABILITIES` (`RFDS_NO`) to 1
  since it assumes that the fleet only consists of processors that are not
  affected by RFDS.
- [#4388](https://github.com/firecracker-microvm/firecracker/pull/4388): Avoid
  setting `kvm_immediate_exit` to 1 if are already handling an exit, or if the
  vCPU is stopped. This avoids a spurious KVM exit upon restoring snapshots.
- [#4567](https://github.com/firecracker-microvm/firecracker/pull/4567): Do not
  initialize vCPUs in powered-off state upon snapshot restore. No functional
  change, as vCPU initialization is only relevant for the booted case (where the
  guest expects CPUs to be powered off).

### Deprecated

- Firecracker's `--start-time-cpu-us` and `--start-time-us` parameters are
  deprecated and will be removed in v2.0 or later. They are used by the jailer
  to pass the value that should be subtracted from the (CPU) time, when emitting
  the `start_time_us` and `start_time_cpu_us` metrics. These parameters were
  never meant to be used by end customers, and we recommend doing any such time
  adjustments outside Firecracker.
- Booting with microVM kernels that rely on MPTable on x86_64 is deprecated and
  support will be removed in v2.0 or later. We suggest to users of Firecracker
  to use guest kernels with ACPI support. For x86_64 microVMs, ACPI will be the
  only way Firecracker passes hardware information to the guest once MPTable
  support is removed.

### Fixed

- [#4526](https://github.com/firecracker-microvm/firecracker/pull/4526): Added a
  check in the network TX path that the size of the network frames the guest
  passes to us is not bigger than the maximum frame the device expects to
  handle. On the TX path, we copy frames destined to MMDS from guest memory to
  Firecracker memory. Without the check, a mis-behaving virtio-net driver could
  cause an increase in the memory footprint of the Firecracker process. Now, if
  we receive such a frame, we ignore it and increase `Net::tx_malformed_frames`
  metric.
- [#4536](https://github.com/firecracker-microvm/firecracker/pull/4536): Make
  the first differential snapshot taken after a full snapshot contain only the
  set of memory pages changed since the full snapshot. Previously, these
  differential snapshots would contain all memory pages. This will result in
  potentially much smaller differential snapshots after a full snapshot.
- [#4578](https://github.com/firecracker-microvm/firecracker/pull/4578): Fix
  UFFD support not being forward-compatible with new ioctl options introduced in
  Linux 6.6. See also
  <https://github.com/bytecodealliance/userfaultfd-rs/issues/61>.
- [#4618](https://github.com/firecracker-microvm/firecracker/pull/4618): On
  x86_64, when taking a snapshot, if a vCPU has MSR_IA32_TSC_DEADLINE set to 0,
  Firecracker will replace it with the MSR_IA32_TSC value from the same vCPU.
  This is to guarantee that the vCPU will continue receiving TSC interrupts
  after restoring from the snapshot even if an interrupt is lost when taking a
  snapshot.
- [#4666](https://github.com/firecracker-microvm/firecracker/pull/4666): Fixed
  Firecracker sometimes restoring `MSR_IA32_TSC_DEADLINE` before `MSR_IA32_TSC`.
  Now it always restores `MSR_IA32_TSC_DEADLINE` MSR after `MSR_IA32_TSC`, as
  KVM relies on the guest TSC for correct restoration of
  `MSR_IA32_TSC_DEADLINE`. This fixed guests using the `TSC_DEADLINE` hardware
  feature receiving incorrect timer interrupts after snapshot restoration, which
  could lead to them seemingly getting stuck in sleep-related syscalls (see also
  <https://github.com/firecracker-microvm/firecracker/pull/4099>).

## [1.7.0]

### Added

- [#4346](https://github.com/firecracker-microvm/firecracker/pull/4346): Added
  support to emit aggregate (minimum/maximum/sum) latency for
  `VcpuExit::MmioRead`, `VcpuExit::MmioWrite`, `VcpuExit::IoIn` and
  `VcpuExit::IoOut`. The average for these VM exits is not emitted since it can
  be deduced from the available emitted metrics.
- [#4360](https://github.com/firecracker-microvm/firecracker/pull/4360): Added
  dev-preview support for backing a VM's guest memory by 2M hugetlbfs pages.
  Please see the [documentation](docs/hugepages.md) for more information
- [#4486](https://github.com/firecracker-microvm/firecracker/pull/4486): Added
  block and net device metrics for file/tap access latencies and queue backlog
  lengths, which can be used to analyse saturation of the Firecracker VMM thread
  and underlying layers. Queue backlog length metrics are flushed periodically.
  They can be used to esimtate an average queue length by request by dividing
  its value by the number of requests served.

### Changed

- [#4230](https://github.com/firecracker-microvm/firecracker/pull/4230): Changed
  microVM snapshot format version strategy. Firecracker snapshot format now has
  a version that is independent of Firecracker version. The current version of
  the snapshot format is v1.0.0. From now on, the Firecracker binary will define
  the snapshot format version it supports and it will only be able to load
  snapshots with format that is backwards compatible with that version. Users
  can pass the `--snapshot-version` flag to the Firecracker binary to see its
  supported snapshot version format. This change renders all previous
  Firecracker snapshots (up to Firecracker version v1.6.0) incompatible with the
  current Firecracker version.

- [#4449](https://github.com/firecracker-microvm/firecracker/pull/4449): Added
  information about page size to the payload Firecracker sends to the UFFD
  handler. Each memory region object now contains a `page_size_kib` field. See
  also the [hugepages documentation](docs/hugepages.md).

- [#4498](https://github.com/firecracker-microvm/firecracker/pull/4498): Only
  use memfd to back guest memory if a vhost-user-blk device is configured,
  otherwise use anonymous private memory. This is because serving page faults of
  shared memory used by memfd is slower and may impact workloads.

### Fixed

- [#4409](https://github.com/firecracker-microvm/firecracker/pull/4409): Fixed a
  bug in the cpu-template-helper that made it panic during conversion of cpu
  configuration with SVE registers to the cpu template on aarch64 platform. Now
  cpu-template-helper will print warnings if it encounters SVE registers during
  the conversion process. This is because cpu templates are limited to only
  modify registers less than 128 bits.
- [#4413](https://github.com/firecracker-microvm/firecracker/pull/4413): Fixed a
  bug in the Firecracker that prevented it to restore snapshots of VMs that had
  SVE enabled.
- [#4414](https://github.com/firecracker-microvm/firecracker/pull/4360): Made
  `PATCH` requests to the `/machine-config` endpoint transactional, meaning
  Firecracker's configuration will be unchanged if the request returns an error.
  This fixes a bug where a microVM with incompatible balloon and guest memory
  size could be booted, due to the check for this condition happening after
  Firecracker's configuration was updated.
- [#4259](https://github.com/firecracker-microvm/firecracker/pull/4259): Added a
  double fork mechanism in the Jailer to avoid setsid() failures occurred while
  running Jailer as the process group leader. However, this changed the
  behaviour of Jailer and now the Firecracker process will always have a
  different PID than the Jailer process.
  [#4436](https://github.com/firecracker-microvm/firecracker/pull/4436): Added a
  "Known Limitations" section in the Jailer docs to highlight the above change
  in behaviour introduced in PR#4259.
  [#4442](https://github.com/firecracker-microvm/firecracker/pull/4442): As a
  solution to the change in behaviour introduced in PR#4259, provided a
  mechanism to reliably fetch Firecracker PID. With this change, Firecracker
  process's PID will always be available in the Jailer's root directory
  regardless of whether new_pid_ns was set.
- [#4468](https://github.com/firecracker-microvm/firecracker/pull/4468): Fixed a
  bug where a client would hang or timeout when querying for an MMDS path whose
  content is empty, because the 'Content-Length' header field was missing in a
  response.

## [1.6.0]

### Added

- [#4145](https://github.com/firecracker-microvm/firecracker/pull/4145): Added
  support for per net device metrics. In addition to aggregate metrics `net`,
  each individual net device will emit metrics under the label
  `"net_{iface_id}"`. E.g. the associated metrics for the endpoint
  `"/network-interfaces/eth0"` will be available under `"net_eth0"` in the
  metrics json object.
- [#4202](https://github.com/firecracker-microvm/firecracker/pull/4202): Added
  support for per block device metrics. In addition to aggregate metrics
  `block`, each individual block device will emit metrics under the label
  `"block_{drive_id}"`. E.g. the associated metrics for the endpoint
  `"/drives/{drive_id}"` will be available under `"block_drive_id"` in the
  metrics json object.
- [#4205](https://github.com/firecracker-microvm/firecracker/pull/4205): Added a
  new `vm-state` subcommand to `info-vmstate` command in the `snapshot-editor`
  tool to print MicrovmState of vmstate snapshot file in a readable format. Also
  made the `vcpu-states` subcommand available on x86_64.
- [#4063](https://github.com/firecracker-microvm/firecracker/pull/4063): Added
  source-level instrumentation based tracing. See [tracing](./docs/tracing.md)
  for more details.
- [#4138](https://github.com/firecracker-microvm/firecracker/pull/4138),
  [#4170](https://github.com/firecracker-microvm/firecracker/pull/4170),
  [#4223](https://github.com/firecracker-microvm/firecracker/pull/4223),
  [#4247](https://github.com/firecracker-microvm/firecracker/pull/4247),
  [#4226](https://github.com/firecracker-microvm/firecracker/pull/4226): Added
  **developer preview only** (NOT for production use) support for vhost-user
  block devices. Firecracker implements a vhost-user frontend. Users are free to
  choose from existing open source backend solutions or their own
  implementation. Known limitation: snapshotting is not currently supported for
  microVMs containing vhost-user block devices. See the
  [related doc page](./docs/api_requests/block-vhost-user.md) for details. The
  device emits metrics under the label `"vhost_user_{device}_{drive_id}"`.

### Changed

- [#4309](https://github.com/firecracker-microvm/firecracker/pull/4309): The
  jailer's option `--parent-cgroup` will move the process to that cgroup if no
  `cgroup` options are provided.
- Simplified and clarified the removal policy of deprecated API elements to
  follow semantic versioning 2.0.0. For more information, please refer to
  [this GitHub discussion](https://github.com/firecracker-microvm/firecracker/discussions/4135).
- [#4180](https://github.com/firecracker-microvm/firecracker/pull/4180):
  Refactored error propagation to avoid logging and printing an error on exits
  with a zero exit code. Now, on successful exit "Firecracker exited
  successfully" is logged.
- [#4194](https://github.com/firecracker-microvm/firecracker/pull/4194): Removed
  support for creating Firecracker snapshots targeting older versions of
  Firecracker. With this change, running 'firecracker --version' will not print
  the supported snapshot versions.
- [#4301](https://github.com/firecracker-microvm/firecracker/pull/4301): Allow
  merging of diff snapshots into base snapshots by directly writing the diff
  snapshot on top of the base snapshot's memory file. This can be done by
  setting the `mem_file_path` to the path of the pre-existing full snapshot.

### Deprecated

- [#4209](https://github.com/firecracker-microvm/firecracker/pull/4209):
  `rebase-snap` tool is now deprecated. Users should use `snapshot-editor` for
  rebasing diff snapshots.

### Fixed

- [#4171](https://github.com/firecracker-microvm/firecracker/pull/4171): Fixed a
  bug that ignored the `--show-log-origin` option, preventing it from printing
  the source code file of the log messages.
- [#4178](https://github.com/firecracker-microvm/firecracker/pull/4178): Fixed a
  bug reporting a non-zero exit code on successful shutdown when starting
  Firecracker with `--no-api`.
- [#4261](https://github.com/firecracker-microvm/firecracker/pull/4261): Fixed a
  bug where Firecracker would log "RunWithApiError error: MicroVMStopped without
  an error: GenericError" when exiting after encountering an emulation error. It
  now correctly prints "RunWithApiError error: MicroVMStopped _with_ an error:
  GenericError".
- [#4242](https://github.com/firecracker-microvm/firecracker/pull/4242): Fixed a
  bug introduced in #4047 that limited the `--level` option of logger to
  Pascal-cased values (e.g. accepting "Info", but not "info"). It now ignores
  case again.
- [#4286](https://github.com/firecracker-microvm/firecracker/pull/4286): Fixed a
  bug in the asynchronous virtio-block engine that rendered the device
  non-functional after a PATCH request was issued to Firecracker for updating
  the path to the host-side backing file of the device.
- [#4301](https://github.com/firecracker-microvm/firecracker/pull/4301): Fixed a
  bug where if Firecracker was instructed to take a snapshot of a microvm which
  itself was restored from a snapshot, specifying `mem_file_path` to be the path
  of the memory file from which the microvm was restored would result in both
  the microvm and the snapshot being corrupted. It now instead performs a
  "write-back" of all memory that was updated since the snapshot was originally
  loaded.

## [1.5.0]

### Added

- [#3837](https://github.com/firecracker-microvm/firecracker/issues/3837): Added
  official support for Linux 6.1. See
  [prod-host-setup](./docs/prod-host-setup.md) for some security and performance
  considerations.
- [#4045](https://github.com/firecracker-microvm/firecracker/pull/4045) and
  [#4075](https://github.com/firecracker-microvm/firecracker/pull/4075): Added
  `snapshot-editor` tool for modifications of snapshot files. It allows for
  rebasing of memory snapshot files, printing and removing aarch64 registers
  from the vmstate and obtaining snapshot version.
- [#3967](https://github.com/firecracker-microvm/firecracker/pull/3967/): Added
  new fields to the custom CPU templates. (aarch64 only) `vcpu_features` field
  allows modifications of vCPU features enabled during vCPU initialization.
  `kvm_capabilities` field allows modifications of KVM capability checks that
  Firecracker performs during boot. If any of these fields are in use, minimal
  target snapshot version is restricted to 1.5.

### Changed

- Updated deserialization of `bitmap` for custom CPU templates to allow usage of
  '\_' as a separator.
- Changed the strip feature of `cpu-template-helper` tool to operate bitwise.
- Better logs during validation of CPU ID in snapshot restoration path. Also
  Firecracker now does not fail if it can't get CPU ID from the host or can't
  find CPU ID in the snapshot.
- Changed the serial device to only try to initialize itself if stdin is a
  terminal or a FIFO pipe. This fixes logged warnings about the serial device
  failing to initialize if the process is daemonized (in which case stdin is
  /dev/null instead of a terminal).
- Changed to show a warning message when launching a microVM with C3 template on
  a processor prior to Intel Cascade Lake, because the guest kernel does not
  apply the mitigation against MMIO stale data vulnerability when it is running
  on a processor that does not enumerate FBSDP_NO, PSDP_NO and SBDR_SSDP_NO on
  IA32_ARCH_CAPABILITIES MSR.
- Made Firecracker resize its file descriptor table on process start. It now
  preallocates the in-kernel fdtable to hold `RLIMIT_NOFILE` many fds (or 2048
  if no limit is set). This avoids the kernel reallocating the fdtable during
  Firecracker operations, resulting in a 30ms to 70ms reduction of snapshot
  restore times for medium to large microVMs with many devices attached.
- Changed the dump feature of `cpu-template-helper` tool not to enumerate
  program counter (PC) on ARM because it is determined by the given kernel image
  and it is useless in the custom CPU template context.
- The ability to create snapshots for an older version of Firecracker is now
  deprecated. As a result, the `version` body field in `PUT` on
  `/snapshot/create` request in deprecated.
- Added support for the /dev/userfaultfd device available on linux kernels >=
  6.1. This is the default for creating UFFD handlers on these kernel versions.
  If it is unavailable, Firecracker falls back to the userfaultfd syscall.
- Deprecated `cpu_template` field in `PUT` and `PATCH` requests on
  `/machine-config` API, which is used to set a static CPU template. Custom CPU
  templates added in v1.4.0 are available as an improved iteration of the static
  CPU templates. For more information about the transition from static CPU
  templates to custom CPU templates, please refer to
  [this GitHub discussion](https://github.com/firecracker-microvm/firecracker/discussions/4135).
- Changed default log level from
  [`Warn`](https://docs.rs/log/latest/log/enum.Level.html#variant.Warn) to
  [`Info`](https://docs.rs/log/latest/log/enum.Level.html#variant.Info). This
  results in more logs being output by default.

### Fixed

- Fixed a change in behavior of normalize host brand string that breaks
  Firecracker on external instances.
- Fixed the T2A CPU template not to unset the MMX bit (CPUID.80000001h:EDX[23])
  and the FXSR bit (CPUID.80000001h:EDX[24]).
- Fixed the T2A CPU template to set the RstrFpErrPtrs bit
  (CPUID.80000008h:EBX[2]).
- Fixed a bug where Firecracker would crash during boot if a guest set up a
  virtio queue that partially overlapped with the MMIO gap. Now Firecracker
  instead correctly refuses to activate the corresponding virtio device.
- Fixed the T2CL CPU template to pass through security mitigation bits that are
  listed by KVM as bits able to be passed through. By making the most use of the
  available hardware security mitigations on a processor that a guest is running
  on, the guest might be able to benefit from performance improvements.
- Fixed the T2S CPU template to set the GDS_NO bit of the IA32_ARCH_CAPABILITIES
  MSR to 1 in accordance with an Intel microcode update. To use the template
  securely, users should apply the latest microcode update on the host.
- Fixed the spelling of the `nomodule` param passed in the default kernel
  command line parameters. This is a **breaking change** for setups that use the
  default kernel command line which also depend on being able to load kernel
  modules at runtime. This may also break setups which use the default kernel
  command line and which use an init binary that inadvertently depends on the
  misspelled param ("nomodules") being present at the command line, since this
  param will no longer be passed.

## [1.4.0]

### Added

- Added support for custom CPU templates allowing users to adjust vCPU features
  exposed to the guest via CPUID, MSRs and ARM registers.
- Introduced V1N1 static CPU template for ARM to represent Neoverse V1 CPU as
  Neoverse N1.
- Added support for the `virtio-rng` entropy device. The device is optional. A
  single device can be enabled per VM using the `/entropy` endpoint.
- Added a `cpu-template-helper` tool for assisting with creating and managing
  custom CPU templates.

### Changed

- Set FDP_EXCPTN_ONLY bit (CPUID.7h.0:EBX[6]) and ZERO_FCS_FDS bit
  (CPUID.7h.0:EBX[13]) in Intel's CPUID normalization process.

### Fixed

- Fixed feature flags in T2S CPU template on Intel Ice Lake.
- Fixed CPUID leaf 0xb to be exposed to guests running on AMD host.
- Fixed a performance regression in the jailer logic for closing open file
  descriptors. Related to:
  [#3542](https://github.com/firecracker-microvm/firecracker/issues/3542).
- A race condition that has been identified between the API thread and the VMM
  thread due to a misconfiguration of the `api_event_fd`.
- Fixed CPUID leaf 0x1 to disable perfmon and debug feature on x86 host.
- Fixed passing through cache information from host in CPUID leaf 0x80000006.
- Fixed the T2S CPU template to set the RRSBA bit of the IA32_ARCH_CAPABILITIES
  MSR to 1 in accordance with an Intel microcode update.
- Fixed the T2CL CPU template to pass through the RSBA and RRSBA bits of the
  IA32_ARCH_CAPABILITIES MSR from the host in accordance with an Intel microcode
  update.
- Fixed passing through cache information from host in CPUID leaf 0x80000005.
- Fixed the T2A CPU template to disable SVM (nested virtualization).
- Fixed the T2A CPU template to set EferLmsleUnsupported bit
  (CPUID.80000008h:EBX[20]), which indicates that EFER[LMSLE] is not supported.

## [1.3.0]

### Added

- Introduced T2CL (Intel) and T2A (AMD) CPU templates to provide instruction set
  feature parity between Intel and AMD CPUs when using these templates.
- Added Graviton3 support (c7g instance type).

### Changed

- Improved error message when invalid network backend provided.
- Improved TCP throughput by between 5% and 15% (depending on CPU) by using
  scatter-gather I/O in the net device's TX path.
- Upgraded Rust toolchain from 1.64.0 to 1.66.0.
- Made seccompiler output bit-reproducible.

### Fixed

- Fixed feature flags in T2 CPU template on Intel Ice Lake.

## [1.2.0]

### Added

- Added a new CPU template called `T2S`. This exposes the same CPUID as `T2` to
  the Guest and also overwrites the `ARCH_CAPABILITIES` MSR to expose a reduced
  set of capabilities. With regards to hardware vulnerabilities and mitigations,
  the Guest vCPU will apear to look like a Skylake CPU, making it safe to
  snapshot uVMs running on a newer host CPU (Cascade Lake) and restore on a host
  that has a Skylake CPU.
- Added a new CLI option `--metrics-path PATH`. It accepts a file parameter
  where metrics will be sent to.
- Added baselines for m6i.metal and m6a.metal for all long running performance
  tests.
- Releases now include debuginfo files.

### Changed

- Changed the jailer option `--exec-file` to fail if the filename does not
  contain the string `firecracker` to prevent from running non-firecracker
  binaries.
- Upgraded Rust toolchain from 1.52.1 to 1.64.0.
- Switched to specifying our dependencies using caret requirements instead of
  comparison requirements.
- Updated all dependencies to their respective newest versions.

### Fixed

- Made the `T2` template more robust by explicitly disabling additional CPUID
  flags that should be off but were missed initially or that were not available
  in the spec when the template was created.
- Now MAC address is correctly displayed when queried with GET `/vm/config` if
  left unspecified in both pre and post snapshot states.
- Fixed a self-DoS scenario in the virtio-queue code by reporting and
  terminating execution when the number of available descriptors reported by the
  driver is higher than the queue size.
- Fixed the bad handling of kernel cmdline parameters when init arguments were
  provided in the `boot_args` field of the JSON body of the PUT `/boot-source`
  request.
- Fixed a bug on ARM64 hosts where the upper 64bits of the V0-V31 FL/SIMD
  registers were not saved correctly when taking a snapshot, potentially leading
  to data loss. This change invalidates all ARM64 snapshots taken with versions
  of Firecracker \<= 1.1.3.
- Improved stability and security when saving CPU MSRs in snapshots.

## [1.1.0]

### Added

- The API `PATCH` methods for `machine-config` can now be used to reset the
  `cpu_template` to `"None"`. Until this change there was no way to reset the
  `cpu_template` once it was set.
- Added a `rebase-snap` tool for rebasing a diff snapshot over a base snapshot.
- Mmds version is persisted across snapshot-restore. Snapshot compatibility is
  preserved bidirectionally, to and from a Firecracker version that does not
  support persisting the Mmds version. In such cases, the default V1 option is
  used.
- Added `--mmds-size-limit` for limiting the mmds data store size instead of
  piggy-backing on `--http-api-max-payload-size`. If left unconfigured it
  defaults to the value of `--http-api-max-payload-size`, to provide backwards
  compatibility.
- Added optional `mem_backend` body field in `PUT` requests on `/snapshot/load`.
  This new parameter is an object that defines the configuration of the backend
  responsible for handling memory loading during snapshot restore. The
  `mem_backend` parameter contains `backend_type` and `backend_path` required
  fields. `backend_type` is an enum that can take either `File` or `Uffd` as
  value. Interpretation of `backend_path` field depends on the value of
  `backend_type`. If `File`, then the user must provide the path to file that
  contains the guest memory to be loaded. Otherwise, if `backend_type` is
  `Uffd`, then `backend_path` is the path to a unix domain socket where a custom
  page fault handler process is listening and expecting a UFFD to be sent by
  Firecracker. The UFFD is used to handle the guest memory page faults in the
  separate process.
- Added logging for the snapshot/restore and async block device IO engine
  features to indicate they are in development preview.

### Changed

- The API `PATCH` method for `/machine-config` can be now used to change
  `track_dirty_pages` on aarch64.
- MmdsV2 is now Generally Available.
- MmdsV1 is now deprecated and will be removed in Firecracker v2.0.0. Use MmdsV2
  instead.
- Deprecated `mem_file_path` body field in `PUT` on `/snapshot/load` request.

### Fixed

- Fixed inconsistency that allowed the start of a microVM from a JSON file
  without specifying the `vcpu_count` and `mem_size_mib` parameters for
  `machine-config` although they are mandatory when configuring via the API. Now
  these fields are mandatory when specifying `machine-config` in the JSON file
  and when using the `PUT` request on `/machine-config`.
- Fixed inconsistency that allowed a user to specify the `cpu_template`
  parameter and set `smt` to `True` in `machine-config` when starting from a
  JSON file on aarch64 even though they are not permitted when using `PUT` or
  `PATCH` in the API. Now Firecracker will return an error on aarch64 if `smt`
  is set to `True` or if `cpu_template` is specified.
- Fixed inconsistent behaviour of the `PUT` method for `/machine-config` that
  would reset the `track_dirty_pages` parameter to `false` if it was not
  specified in the JSON body of the request, but left the `cpu_template`
  parameter intact if it was not present in the request. Now a `PUT` request for
  `/machine-config` will reset all optional parameters (`smt`, `cpu_template`,
  `track_dirty_pages`) to their default values if they are not specified in the
  `PUT` request.
- Fixed incosistency in the swagger definition with the current state of the
  `/vm/config` endpoint.

## [1.0.0]

### Added

- Added jailer option `--parent-cgroup <relative_path>` to allow the placement
  of microvm cgroups in custom cgroup nested hierarchies. The default value is
  `<exec-file>` which is backwards compatible to the behavior before this
  change.
- Added jailer option `--cgroup-version <1|2>` to support running the jailer on
  systems that have cgroup-v2. Default value is `1` which means that if
  `--cgroup-version` is not specified, the jailer will try to create cgroups on
  cgroup-v1 hierarchies only.
- Added `--http-api-max-payload-size` parameter to configure the maximum payload
  size for PUT and PATCH requests.
- Limit MMDS data store size to `--http-api-max-payload-size`.
- Cleanup all environment variables in Jailer.
- Added metrics for accesses to deprecated HTTP and command line API endpoints.
- Added permanent HTTP endpoint for `GET` on `/version` for getting the
  Firecracker version.
- Added `--metadata` parameter to enable MMDS content to be supplied from a file
  allowing the MMDS to be used when using `--no-api` to disable the API server.
- Checksum file for the release assets.
- Added support for custom headers to MMDS requests. Accepted headers are:
  `X-metadata-token`, which accepts a string value that provides a session token
  for MMDS requests; and `X-metadata-token-ttl-seconds`, which specifies the
  lifetime of the session token in seconds.
- Support and validation for host and guest kernel 5.10.
- A [kernel support policy](docs/kernel-policy.md).
- Added `io_engine` to the pre-boot block device configuration. Possible values:
  `Sync` (the default option) or `Async` (only available for kernels newer than
  5.10.51). The `Async` variant introduces a block device engine that uses
  io_uring for executing requests asynchronously, which is in **developer
  preview** (NOT for production use). See
  `docs/api_requests/block-io-engine.md`.
- Added `block.io_engine_throttled_events` metric for measuring the number of
  virtio events throttled because of the IO engine.
- New optional `version` field to PUT requests towards `/mmds/config` to
  configure MMDS version. Accepted values are `V1` and `V2` and default is `V1`.
  MMDS `V2` is **developer preview only** (NOT for production use) and it does
  not currently work after snapshot load.
- Mandatory `network_interfaces` field to PUT requests towards `/mmds/config`
  which contains a list of network interface IDs capable of forwarding packets
  to MMDS.

### Changed

- Removed the `--node` jailer parameter.
- Deprecated `vsock_id` body field in `PUT`s on `/vsock`.
- Removed the deprecated the `--seccomp-level parameter`.
- `GET` requests to MMDS require a session token to be provided through
  `X-metadata-token` header when using V2.
- Allow `PUT` requests to MMDS in order to generate a session token to be used
  for future `GET` requests when version 2 is used.
- Remove `allow_mmds_requests` field from the request body that attaches network
  interfaces. Specifying interfaces that allow forwarding requests to MMDS is
  done by adding the network interface's ID to the `network_interfaces` field of
  PUT `/mmds/config` request's body.
- Renamed `/machine-config` `ht_enabled` to `smt`.
- `smt` field is now optional on PUT `/machine-config`, defaulting to `false`.
- Configuring `smt: true` on aarch64 via the API is forbidden.

### Fixed

- GET `/vm/config` was returning a default config object after restoring from a
  snapshot. It now correctly returns the config of the original microVM, except
  for boot_config and the cpu_template and smt fields of the machine config,
  which are currently lost.
- Fixed incorrect propagation of init parameters in kernel commandline. Related
  to: [#2709](https://github.com/firecracker-microvm/firecracker/issues/2709).
- Adapt T2 and C3 CPU templates for kernel 5.10. Firecracker was not previously
  masking some CPU features of the host or emulated by KVM, introduced in more
  recent kernels: `umip`, `vmx`, `avx512_vnni`.
- Fix jailer's cgroup implementation to accept properties that contain multiple
  dots.

## [0.25.0]

### Added

- Added devtool build `--ssh-keys` flag to support fetching from private git
  repositories.
- Added option to configure block device flush.
- Added `--new-pid-ns` flag to the Jailer in order to spawn the Firecracker
  process in a new PID namespace.
- Added API metrics for `GET`, `PUT` and `PATCH` requests on `/mmds` endpoint.
- Added `--describe-snapshot` flag to Firecracker to fetch the data format
  version of a snapshot state file provided as argument.
- Added `--no-seccomp` parameter for disabling the default seccomp filters.
- Added `--seccomp-filter` parameter for supplying user-provided, custom
  filters.
- Added the `seccompiler-bin` binary that is used to compile JSON seccomp
  filters into serialized BPF for Firecracker consumption.
- Snapshotting support for GICv2 enabled guests.
- Added `devtool install` to deploy built binaries in `/usr/local/bin` or a
  given path.
- Added code logic to send `VIRTIO_VSOCK_EVENT_TRANSPORT_RESET` on snapshot
  creation, when the Vsock device is active. The event will close active
  connections on the guest.
- Added `GET` request on `/vm/config` that provides full microVM configuration
  as a JSON HTTP response.
- Added `--resource-limit` flag to jailer to limit resources such as: number of
  file descriptors allowed at a time (with a default value of 2048) and maximum
  size of files created by the process.

### Changed

- Changed Docker images repository from DockerHub to Amazon ECR.
- Fixed off-by-one error in virtio-block descriptor address validation.
- Changed the `PATCH` request on `/balloon/statistics` to schedule the first
  statistics update immediately after processing the request.
- Deprecated the `--seccomp-level parameter`. It will be removed in a future
  release. Using it logs a runtime warning.
- Experimental gnu libc builds use empty default seccomp filters, allowing all
  system calls.

### Fixed

- Fixed non-compliant check for the RTC device ensuring a fixed 4-sized data
  buffer.
- Unnecessary interrupt assertion was removed from the RTC. However, a dummy
  interrupt is still allocated for snapshot compatibility reasons.
- Fixed the SIGPIPE signal handler so Firecracker no longer exits. The signal is
  still recorded in metrics and logs.
- Fixed ballooning API definitions by renaming all fields which mentioned "MB"
  to use "MiB" instead.
- Snapshot related host files (vm-state, memory, block backing files) are now
  flushed to their backing mediums as part of the CreateSnapshot operation.
- Fixed the SSBD mitigation not being enabled on `aarch64` with the provided
  `prod-host-setup.md`.
- Fixed the balloon statistics not working after a snapshot restore event.
- The `utc_timestamp_ms` now reports the timestamp in ms from the UTC UNIX
  Epoch, as the name suggests. It was previously using a monotonic clock with an
  undefined starting point.

## [0.24.0]

### Added

- Added optional `resume_vm` field to `/snapshot/load` API call.
- Added support for block rate limiter PATCH.
- Added devtool test `-c|--cpuset-cpus` flag for cpus confinement when tests
  run.
- Added devtool test `-m|--cpuset-mems` flag for memory confinement when tests
  run.
- Added the virtio traditional memory ballooning device.
- Added a mechanism to handle vCPU/VMM errors that result in process
  termination.
- Added incremental guest memory snapshot support.
- Added aarch64 snapshot support.

### Changed

- Change the information provided in `DescribeInstance` command to provide
  microVM state information (Not started/Running/Paused) instead of whether it's
  started or not.
- Removed the jailer `--extra-args` parameter. It was a noop, having been
  replaced by the `--` separator for extra arguments.
- Changed the output of the `--version` command line parameter to include a list
  of supported snapshot data format versions for the firecracker binary.
- Increased the maximum number of virtio devices from 11 to 19.
- Added a new check that prevents creating v0.23 snapshots when more than 11
  devices are attached.
- If the stdout buffer is full and non-blocking, the serial writes no longer
  block. Any new bytes will be lost, until the buffer is freed. The device also
  logs these errors and increments the `uart.error_count` metric for each lost
  byte.

### Fixed

- Fixed inconsistency in YAML file InstanceInfo definition

## [0.23.0]

### Added

- Added metric for throttled block device events.
- Added metrics for counting rate limiter throttling events.
- Added metric for counting MAC address updates.
- Added metrics for counting TAP read and write errors.
- Added metrics for counting RX and TX partial writes.
- Added metrics that measure the duration of pausing and resuming the microVM,
  from the VMM perspective.
- Added metric for measuring the duration of the last full/diff snapshot
  created, from the VMM perspective.
- Added metric for measuring the duration of loading a snapshot, from the VMM
  perspective.
- Added metrics that measure the duration of pausing and resuming the microVM,
  from the API (user) perspective.
- Added metric for measuring the duration of the last full/diff snapshot
  created, from the API (user) perspective.
- Added metric for measuring the duration of loading a snapshot, from the API
  (user) perspective.
- Added `track_dirty_pages` field to `machine-config`. If enabled, Firecracker
  can create incremental guest memory snapshots by saving the dirty guest pages
  in a sparse file.
- Added a new API call, `PATCH /vm`, for changing the microVM state (to `Paused`
  or `Resumed`).
- Added a new API call, `PUT /snapshot/create`, for creating a full or diff
  snapshot.
- Added a new API call, `PUT /snapshot/load`, for loading a snapshot.
- Added new jailer command line argument `--cgroup` which allow the user to
  specify the cgroups that are going to be set by the Jailer.
- Added full support for AMD CPUs (General Availability). More details
  [here](README.md#supported-platforms).

### Fixed

- Boot time on AMD achieves the desired performance (i.e under 150ms).

### Changed

- The logger `level` field is now case-insensitive.
- Disabled boot timer device after restoring a snapshot.
- Enabled boot timer device only when specifically requested, by using the
  `--boot-timer` dedicated cmdline parameter.
- firecracker and jailer `--version` now gets updated on each devtool build to
  the output of `git describe --dirty`, if the git repo is available.
- MicroVM process is only attached to the cgroups defined by using `--cgroups`
  or the ones defined indirectly by using `--node`.
- Changed `devtool build` to build jailer binary for `musl` only targets.
  Building jailer binary for `non-musl` targets have been removed.

## [0.22.0]

### Added

- Added a new API call, `PUT /metrics`, for configuring the metrics system.
- Added `app_name` field in InstanceInfo struct for storing the application
  name.
- New command-line parameters for `firecracker`, named `--log-path`, `--level`,
  `--show-level` and `--show-log-origin` that can be used for configuring the
  Logger when starting the process. When using this method for configuration,
  only `--log-path` is mandatory.
- Added a
  [guide](https://github.com/firecracker-microvm/firecracker/blob/v0.22.0/docs/devctr-image.md)
  for updating the dev container image.
- Added a new API call, `PUT /mmds/config`, for configuring the `MMDS` with a
  custom valid link-local IPv4 address.
- Added experimental JSON response format support for MMDS guest applications
  requests.
- Added metrics for the vsock device.
- Added `devtool strip` command which removes debug symbols from the release
  binaries.
- Added the `tx_malformed_frames` metric for the virtio net device, emitted when
  a TX frame missing the VNET header is encountered.

### Fixed

- Added `--version` flag to both Firecracker and Jailer.
- Return `405 Method Not Allowed` MMDS response for non HTTP `GET` MMDS requests
  originating from guest.
- Fixed folder permissions in the jail (#1802).
- Any number of whitespace characters are accepted after ":" when parsing HTTP
  headers.
- Potential panic condition caused by the net device expecting to find a VNET
  header in every frame.
- Potential crash scenario caused by "Content-Length" HTTP header field
  accepting negative values.
- Fixed #1754 - net: traffic blocks when running ingress UDP performance tests
  with very large buffers.

### Changed

- Updated CVE-2019-3016 mitigation information in
  [Production Host Setup](docs/prod-host-setup.md)
- In case of using an invalid JSON as a 'config-file' for Firecracker, the
  process will exit with return code 152.
- Removed the `testrun.sh` wrapper.
- Removed `metrics_fifo` field from the logger configuration.
- Renamed `log_fifo` field from LoggerConfig to `log_path` and `metrics_fifo`
  field from MetricsConfig to `metrics_path`.
- `PATCH /drives/{id}` only allowed post-boot. Use `PUT` for pre-boot updates to
  existing configurations.
- `PATCH /network-interfaces/{id}` only allowed post-boot. Use `PUT` for
  pre-boot updates to existing configurations.
- Changed returned status code from `500 Internal Server Error` to
  `501 Not Implemented`, for queries on the MMDS endpoint in IMDS format, when
  the requested resource value type is unsupported.
- Allowed the MMDS data store to be initialized with all supported JSON types.
  Retrieval of these values within the guest, besides String, Array, and
  Dictionary, is only possible in JSON mode.
- `PATCH` request on `/mmds` before the data store is initialized returns
  `403 BadRequest`.
- Segregated MMDS documentation in MMDS design documentation and MMDS user guide
  documentation.

## [0.21.0]

### Added

- Support for booting with an initial RAM disk image. This image can be
  specified through the new `initrd_path` field of the `/boot-source` API
  request.

### Fixed

- Fixed #1469 - Broken GitHub location for Firecracker release binary.
- The jailer allows changing the default api socket path by using the extra
  arguments passed to firecracker.
- Fixed #1456 - Occasional KVM_EXIT_SHUTDOWN and bad syscall (14) during VM
  shutdown.
- Updated the production host setup guide with steps for addressing
  CVE-2019-18960.
- The HTTP header parsing is now case insensitive.
- The `put_api_requests` and `patch_api_requests` metrics for net devices were
  un-swapped.

### Changed

- Removed redundant `--seccomp-level` jailer parameter since it can be simply
  forwarded to the Firecracker executable using "end of command options"
  convention.
- Removed `memory.dirty_pages` metric.
- Removed `options` field from the logger configuration.
- Decreased release binary size by ~15%.
- Changed default API socket path to `/run/firecracker.socket`. This path also
  applies when running with the jailer.
- Disabled KVM dirty page tracking by default.
- Removed redundant RescanBlockDevice action from the /actions API. The
  functionality is available through the PATCH /drives API. See
  `docs/api_requests/patch-block.md`.

## [0.20.0]

### Added

- Added support for GICv2.

### Fixed

- Fixed CVE-2019-18960 - Fixed a logical error in bounds checking performed on
  vsock virtio descriptors.
- Fixed #1283 - Can't start a VM in AARCH64 with vcpus number more than 16.
- Fixed #1088 - The backtrace are printed on `panic`, no longer causing a
  seccomp fault.
- Fixed #1375 - Change logger options type from `Value` to `Vec<LogOption>` to
  prevent potential unwrap on None panics.
- Fixed #1436 - Raise interrupt for TX queue used descriptors
- Fixed #1439 - Prevent achieving 100% cpu load when the net device rx is
  throttled by the ratelimiter
- Fixed #1437 - Invalid fields in rate limiter related API requests are now
  failing with a proper error message.
- Fixed #1316 - correctly determine the size of a virtio device backed by a
  block device.
- Fixed #1383 - Log failed api requests.

### Changed

- Decreased release binary size by 10%.

## [0.19.0]

### Added

- New command-line parameter for `firecracker`, named `--no-api`, which will
  disable the API server thread. If set, the user won't be able to send any API
  requests, neither before, nor after the vm has booted. It must be paired with
  `--config-file` parameter. Also, when API server is disabled, MMDS is no
  longer available now.
- New command-line parameter for `firecracker`, named `--config-file`, which
  represents the path to a file that contains a JSON which can be used for
  configuring and starting a microVM without sending any API requests.
- The jailer adheres to the "end of command options" convention, meaning all
  parameters specified after `--` are forwarded verbatim to Firecracker.
- Added `KVM_PTP` support to the recommended guest kernel config.
- Added entry in FAQ.md for Firecracker Guest timekeeping.

### Changed

- Vsock API call: `PUT /vsocks/{id}` changed to `PUT /vsock` and no longer
  appear to support multiple vsock devices. Any subsequent calls to this API
  endpoint will override the previous vsock device configuration.
- Removed unused 'Halting' and 'Halted' instance states.
- Vsock host-initiated connections now implement a trivial handshake protocol.
  See the [vsock doc](docs/vsock.md#host-initiated-connections) for details.
  Related to:
  [#1253](https://github.com/firecracker-microvm/firecracker/issues/1253),
  [#1432](https://github.com/firecracker-microvm/firecracker/issues/1432),
  [#1443](https://github.com/firecracker-microvm/firecracker/pull/1443)

### Fixed

- Fixed serial console on aarch64 (GitHub issue #1147).
- Upon panic, the terminal is now reset to canonical mode.
- Explicit error upon failure of vsock device creation.
- The failure message returned by an API call is flushed in the log FIFOs.
- Insert virtio devices in the FDT in order of their addresses sorted from low
  to high.
- Enforce the maximum length of the network interface name to be 16 chars as
  specified in the Linux Kernel.
- Changed the vsock property `id` to `vsock_id` so that the API client can be
  successfully generated from the swagger definition.

## [0.18.0]

### Added

- New device: virtio-vsock, backed by Unix domain sockets (GitHub issue #650).
  See `docs/vsock.md`.

### Changed

- No error is thrown upon a flush metrics intent if logger has not been
  configured.

### Fixed

- Updated the documentation for integration tests.
- Fixed high CPU usage before guest network interface is brought up (GitHub
  issue #1049).
- Fixed an issue that caused the wrong date (month) to appear in the log.
- Fixed a bug that caused the seccomp filter to reject legit syscalls in some
  rare cases (GitHub issue #1206).
- Docs: updated the production host setup guide.
- Docs: updated the rootfs and kernel creation guide.

### Removed

- Removed experimental support for vhost-based vsock devices.

## [0.17.0]

### Added

- New API call: `PATCH /machine-config/`, used to update VM configuration,
  before the microVM boots.
- Added an experimental swagger definition that includes the specification for
  the vsock API call.
- Added a signal handler for `SIGBUS` and `SIGSEGV` that immediately terminates
  the process upon intercepting the signal.
- Added documentation for signal handling utilities.
- Added [alpha] aarch64 support.
- Added metrics for successful read and write operations of MMDS, Net and Block
  devices.

### Changed

- `vcpu_count`, `mem_size_mib` and `ht_enabled` have been changed to be
  mandatory for `PUT` requests on `/machine-config/`.
- Disallow invalid seccomp levels by exiting with error.

### Fixed

- Incorrect handling of bind mounts within the jailed rootfs.
- Corrected the guide for `Alpine` guest setup.

## [0.16.0]

### Added

- Added [alpha] AMD support.
- New `devtool` command: `prepare_release`. This updates the Firecracker
  version, crate dependencies and credits in preparation for a new release.
- New `devtool` command: `tag`. This creates a new git tag for the specified
  release number, based on the changelog contents.
- New doc section about building with glibc.

### Changed

- Dropped the JSON-formatted `context` command-line parameter from Firecracker
  in favor of individual classic command-line parameters.
- When running with `jailer` the location of the API socket has changed to
  `<jail-root-path>/api.socket` (API socket was moved _inside_ the jail).
- `PUT` and `PATCH` requests on `/mmds` with data containing any value type
  other than `String`, `Array`, `Object` will returns status code 400.
- Improved multiple error messages.
- Removed all kernel modules from the recommended kernel config.

### Fixed

- Corrected the seccomp filter when building with glibc.

### Removed

- Removed the `seccomp.bad_syscalls` metric.

## [0.15.2]

### Fixed

- Corrected the conditional compilation of the seccomp rule for `madvise`.

## [0.15.1]

### Fixed

- A `madvise` call issued by the `musl` allocator was added to the seccomp allow
  list to prevent Firecracker from terminating abruptly when allocating memory
  in certain conditions.

## [0.15.0]

### Added

- New API action: SendCtrlAltDel, used to initiate a graceful shutdown, if the
  guest has driver support for i8042 and AT Keyboard. See
  [the docs](docs/api_requests/actions.md#sendctrlaltdel) for details.
- New metric counting the number of egress packets with a spoofed MAC:
  `net.tx_spoofed_mac_count`.
- New API call: `PATCH /network-interfaces/`, used to update the rate limiters
  on a network interface, after the start of a microVM.

### Changed

- Added missing `vmm_version` field to the InstanceInfo API swagger definition,
  and marked several other mandatory fields as such.
- New default command line for guest kernel:
  `reboot=k panic=1 pci=off nomodules 8250.nr_uarts=0 i8042.noaux i8042.nomux i8042.nopnp i8042.dumbkbd`.

### Fixed

- virtio-blk: VIRTIO_BLK_T_FLUSH now working as expected.
- Vsock devices can be attached when starting Firecracker using the jailer.
- Vsock devices work properly when seccomp filtering is enabled.

## [0.14.0]

### Added

- Documentation for development environment setup on AWS in
  `dev-machine-setup.md`.
- Documentation for microVM networking setup in `docs/network-setup.md`.
- Limit the maximum supported vCPUs to 32.

### Changed

- Log the app version when the `Logger` is initialized.
- Pretty print panic information.
- Firecracker terminates with exit code 148 when a syscall which is not present
  in the allow list is intercepted.

### Fixed

- Fixed build with the `vsock` feature.

## [0.13.0]

### Added

- Documentation for Logger API Requests in `docs/api_requests/logger.md`.
- Documentation for Actions API Requests in `docs/api_requests/actions.md`.
- Documentation for MMDS in `docs/mmds.md`.
- Flush metrics on request via a PUT `/actions` with the `action_type` field set
  to `FlushMetrics`.

### Changed

- Updated the swagger definition of the `Logger` to specify the required fields
  and provide default values for optional fields.
- Default `seccomp-level` is `2` (was previously 0).
- API Resource IDs can only contain alphanumeric characters and underscores.

### Fixed

- Seccomp filters are now applied to all Firecracker threads.
- Enforce minimum length of 1 character for the jailer ID.
- Exit with error code when starting the jailer process fails.

### Removed

- Removed `InstanceHalt` from the list of possible actions.

## [0.12.0]

### Added

- The `/logger` API has a new field called `options`. This is an array of
  strings that specify additional logging configurations. The only supported
  value is `LogDirtyPages`.
- When the `LogDirtyPages` option is configured via `PUT /logger`, a new metric
  called `memory.dirty_pages` is computed as the number of pages dirtied by the
  guest since the last time the metric was flushed.
- Log messages on both graceful and forceful termination.
- Availability of the list of dependencies for each commit inside the code base.
- Documentation on vsock experimental feature and host setup recommendations.

### Changed

- `PUT` requests on `/mmds` always return 204 on success.
- `PUT` operations on `/network-interfaces` API resources no longer accept the
  previously required `state` parameter.
- The jailer starts with `--seccomp-level=2` (was previously 0) by default.
- Log messages use `anonymous-instance` as instance id if none is specified.

### Fixed

- Fixed crash upon instance start on hosts without 1GB huge page support.
- Fixed "fault_message" inconsistency between Open API specification and code
  base.
- Ensure MMDS compatibility with C5's IMDS implementation.
- Corrected the swagger specification to ensure `OpenAPI 2.0` compatibility.

## [0.11.0]

### Added

- Apache-2.0 license
- Docs:
  - [charter](CHARTER.md)
  - [contribution guidelines](CONTRIBUTING.md)
  - [design](docs/design.md)
  - [getting started guide](docs/getting-started.md)
  - [security policy](SECURITY.md)
  - [specifications](SPECIFICATION.md)
- **Experimental** vhost-based vsock implementation.

### Changed

- Improved MMDS network stack performance.
- If the logging system is not yet initialized (via `PUT /logger`), log events
  are now sent to stdout/stderr.
- Moved the `instance_info_fails` metric under `get_api_requests`
- Improved [readme](README.md) and added links to more detailed information, now
  featured in subject-specific docs.

### Fixed

- Fixed bug in the MMDS network stack, that caused some RST packets to be sent
  without a destination.
- Fixed bug in `PATCH /drives`, whereby the ID in the path was not checked
  against the ID in the body.

## [0.10.1]

### Fixed

- The Swagger definition was corrected.

## [0.10.0]

### Added

- Each Firecracker process has an associated microVM Metadata Store (MMDS). Its
  contents can be configured using the `/mmds` API resource.

### Changed

- The boot source is specified only with the `kernel_image_path` and the
  optional parameter `boot_args`. All other fields are removed.
- The `path_on_host` property in the drive specification is now marked as
  _mandatory_.
- PATCH drive only allows patching/changing the `path_on_host` property.
- All PUT and PATCH requests return the status code 204.
- CPUID brand string (aka model name) now includes the host CPU frequency.
- API requests which add guest network interfaces have an additional parameter,
  `allow_mmds_requests` which defaults to `false`.
- Stopping the guest (e.g. using the `reboot` command) also terminates the
  Firecracker process. When the Firecracker process ends for any reason, (other
  than `kill -9`), metrics are flushed at the very end.
- On startup `jailer` closes all inherited file descriptors based on
  `sysconf(_SC_OPEN_MAX)` except input, output and error.
- The microVM ID prefixes each Firecracker log line. This ID also appears in the
  process `cmdline` so it's now possible to `ps | grep <ID>` for it.

## [0.9.0]

### Added

- Seccomp filtering is configured via the `--seccomp-level` jailer parameter.
- Firecracker logs the starting addresses of host memory areas provided as guest
  memory slots to KVM.
- The metric `panic_count` gets incremented to signal that a panic has occurred.
- Firecracker logs a backtrace when it crashes following a panic.
- Added basic instrumentation support for measuring boot time.

### Changed

- `StartInstance` is a synchronous API request (it used to be an asynchronous
  request).

### Fixed

- Ensure that fault messages sent by the API have valid JSON bodies.
- Use HTTP response code 500 for internal Firecracker errors, and 400 for user
  errors on InstanceStart.
- Serialize the machine configuration fields to the correct data types (as
  specified in the Swagger definition).
- NUMA node assignment is properly enforced by the jailer.
- The `is_root_device` and `is_read_only` properties are now marked as required
  in the Swagger definition of `Drive` object properties.

### Removed

- `GET` requests on the `/actions` API resource are no longer supported.
- The metrics associated with asynchronous actions have been removed.
- Remove the `action_id` parameter for `InstanceStart`, both from the URI and
  the JSON request body.

## [0.8.0]

### Added

- The jailer can now be configured to enter a preexisting network namespace, and
  to run as a daemon.
- Enabled PATCH operations on `/drives` resources.

### Changed

- The microVM `id` supplied to the jailer may now contain alphanumeric
  characters and hyphens, up to a maximum length of 64 characters.
- Replaced the `permissions` property of `/drives` resources with a boolean.
- Removed the `state` property of `/drives` resources.

## [0.7.0]

### Added

- Rate limiting functionality allows specifying an initial one time burst size.
- Firecracker can now boot from an arbitrary boot partition by specifying its
  unique id in the driver's API call.
- Block device rescan is triggered via a PUT `/actions` with the drive ID in the
  action body's `payload` field and the `action_type` field set to
  `BlockDeviceRescan`.

### Changed

- Removed `noapic` from the default guest kernel command line.
- The `action_id` parameter is no longer required for synchronous PUT requests
  to `/actions`.
- PUT requests are no longer allowed on `/drives` resources after the guest has
  booted.

### Fixed

- Fixed guest instance kernel loader to accelerate vCPUs launch and consequently
  guest kernel boot.
- Fixed network emulation to improve IO performance.

## [0.6.0]

### Added

- Firecracker uses two different named pipes to record human readable logs and
  metrics, respectively.

### Changed

- Seccomp filtering can be enabled via setting the `USE_SECCOMP` environment
  variable.
- It is possible to supply only a partial specification when attaching a rate
  limiter (i.e. just the bandwidth or ops parameter).
- Errors related to guest network interfaces are now more detailed.

### Fixed

- Fixed a bug that was causing Firecracker to panic whenever a `PUT` request was
  sent on an existing network interface.
- The `id` parameter of the `jailer` is required to be an RFC 4122-compliant
  UUID.
- Fixed an issue which caused the network RX rate limiter to be more restrictive
  than intended.
- API requests which contain unknown fields will generate an error.
- Fixed an issue related to high CPU utilization caused by improper `KVM PIT`
  configuration.
- It is now possible to create more than one network tun/tap interface inside a
  jailed Firecracker.

## [0.5.0]

### Added

- Added metrics for API requests, VCPU and device actions for the serial console
  (`UART`), keyboard (`i8042`), block and network devices. Metrics are logged
  every 60 seconds.
- A CPU features template for C3 is available, in addition to the one for T2.
- Seccomp filters restrict Firecracker from calling any other system calls than
  the minimum set it needs to function properly. The filters are enabled by
  setting the `USE_SECCOMP` environment variable to 1 before running
  Firecracker.
- Firecracker can be started by a new binary called `jailer`. The jailer takes
  as command line arguments a unique ID, the path to the Firecracker binary, the
  NUMA node that Firecracker will be assigned to and a `uid` and `gid` for
  Firecracker to run under. It sets up a `chroot` environment and a `cgroup`,
  and calls exec to morph into Firecracker.

### Changed

- In case of failure, the metrics and the panic location are logged before
  aborting.
- Metric values are reset with every flush.
- `CPUTemplate` is now called `CpuTemplate` in order to work seamlessly with the
  swagger code generator for Go.
- `firecracker-beta.yaml` is now called `firecracker.yaml`.

### Fixed

- Handling was added for several untreated KVM exit scenarios, which could have
  led to panic.
- Fixed a bug that caused Firecracker to crash when attempting to disable the
  `IA32_DEBUG_INTERFACE MSR` flag in the T2 CPU features.

### Removed

- Removed a leftover file generated by the logger unit tests.
- Removed `firecracker-v1.0.yaml`.

## [0.4.0]

### Added

- The CPU Template can be set with an API call on `PUT /machine-config`. The
  only available template is T2.
- Hyperthreading can be enabled/disabled with an API call on
  `PUT /machine-config`. By default, hyperthreading is disabled.
- Added boot time performance test (`tests/performance/test_boottime.py`).
- Added Rate Limiter for VirtIO/net and VirtIO/net devices. The Rate Limiter
  uses two token buckets to limit rate on bytes/s and ops/s. The rate limiter
  can be (optionally) configured per drive with a `PUT` on `/drives/{drive_id}`
  and per network interface with a `PUT` on `/network-interface/{iface_id}`.
- Implemented pre-boot PUT updates for `/boot-source`, `/drives`,
  `/network-interfaces` and `/vsock`.
- Added integration tests for `PUT` updates.

### Changed

- Moved the API definition (`swagger/firecracker-beta.yaml`) to the `api_server`
  crate.
- Removed `"console=ttyS0"` and added `"8250.nr_uarts=0"` to the default kernel
  command line to decrease the boot time.
- Changed the CPU topology to have all logical CPUs on a single socket.
- Removed the upper bound on CPU count as with musl there is no good way to get
  the total number of logical processors on a host.
- Build time tests now print the full output of commands.
- Disabled the Performance Monitor Unit and the Turbo Boost.
- Check the expected KVM capabilities before starting the VM.
- Logs now have timestamps.

### Fixed

- `testrun.sh` can run on platforms with more than one package manager by
  setting the package manager via a command line parameter (`-p`).
- Allow correct set up of multiple network-interfaces with auto-generated MAC.
- Fixed sporadic bug in VirtIO which was causing lost packages.
- Don't allow `PUT` requests with empty body on `/machine-config`.
- Deny `PUT` operations after the microvm boots (exception: the temporarily fix
  for live resize of block devices).

### Removed

- Removed examples crate. This used to have a Python example of starting
  Firecracker. This is replaced by `test_api.py` integration tests.
- Removed helper scripts for getting coverage and coding style errors. These
  were replaced by `test_coverage.py` and `test_style.py` test integration
  tests.
- Removed `--vmm-no-api` command line option. Firecracker can only be started
  via the API.

## [0.3.0]

### Added

- Users can interrogate the Machine Configuration (i.e. vcpu count and memory
  size) using a `GET` request on `/machine-config`.
- The logging system can be configured through the API using a `PUT` on
  `/logger`.
- Block devices support live resize by calling `PUT` with the same parameters as
  when the block was created.
- Release builds have Link Time Optimization (LTO) enabled.
- Firecracker is built with `musl`, resulting in a statically linked binary.
- More in-tree integration tests were added as part of the continuous
  integration system.

### Changed

- The vcpu count is enforced to `1` or an even number.
- The Swagger definition of rate limiters was updated.
- Syslog-enabled logs were replaced with a host-file backed mechanism.

### Fixed

- The host topology of the CPU and the caches is not leaked into the microvm
  anymore.
- Boot time was improved by advertising the availability of the TSC deadline
  timer.
- Fixed an issue which prevented Firecracker from working on 4.14 (or newer)
  host kernels.
- Specifying the MAC address for an interface through the API is optional.

### Removed

- Removed support for attaching vsock devices.
- Removed support for building Firecracker with glibc.

## [0.2.0]

### Added

- Users can now interrogate Instance Information (currently just instance state)
  through the API.

### Changed

- Renamed `api/swagger/all.yaml` to `api/swagger/firecracker-v1.0.yaml` which
  specifies targeted API support for Firecracker v1.0.
- Renamed `api/swagger/firecracker-v0.1.yaml` to
  `api/swagger/firecracker-beta.yaml` which specifies the currently supported
  API.
- Users can now enforce that an emulated block device is read-only via the API.
  To specify whether a block device is read-only or read-write, an extra
  "permissions" field was added to the Drive definition in the API. The root
  filesystem is automatically mounted in the guest OS as `ro`/`rw` according to
  the specified "permissions". It's the responsibility of the user to mount any
  other read-only block device as such within the guest OS.
- Users can now stop the guest VM using the API. Actions of type `InstanceHalt`
  are now supported via the API.

### Fixed

- Added support for `getDeviceID()` in `virtIO-block`. Without this, the guest
  Linux kernel would complain at boot time that the operation is unsupported.
- `stdin` control is returned to the Firecracker process when guest VM is
  inactive. Raw mode `stdin` is forwarded to the guest OS when guest VM is
  running.

### Removed

- Removed `api/swagger/actions.yaml`.
- Removed `api/swagger/devices.yaml`.
- Removed `api/swagger/firecracker-mvp.yaml`.
- Removed `api/swagger/limiters.yaml`.

## [0.1.1]

### Changed

- Users can now specify the MAC address of a guest network interface via the
  `PUT` network interface API request. Previously, the guest MAC address
  parameter was ignored.

### Fixed

- Fixed a guest memory allocation issue, which previously led to a potentially
  significant memory chunk being wasted.
- Fixed an issue which caused compilation problems, due to a compatibility
  breaking transitive dependency in the tokio suite of crates.

## [0.1.0]

### Added

- One-process virtual machine manager (one Firecracker per microVM).
- RESTful API running on a unix socket. The API supported by v0.1 can be found
  at `api/swagger/firecracker-v0.1.yaml`.
- Emulated keyboard (`i8042`) and serial console (`UART`). The microVM serial
  console input and output are connected to those of the Firecracker process
  (this allows direct console access to the guest OS).
- The capability of mapping an existing host tun-tap device as a VirtIO/net
  device into the microVM.
- The capability of mapping an existing host file as a GirtIO/block device into
  the microVM.
- The capability of creating a VirtIO/vsock between the host and the microVM.
- Default demand fault paging & CPU oversubscription.
