* [#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.
