firecracker-microvm/firecracker v1.17.0

Firecracker v1.17.0

on GitHub

Added

• #5891: Added support for virtio device reset.
• #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.
• #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, #6017, #6021: Added official support for 6.18 microVM guest kernels.
• #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: 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: 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.
• #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: Added official support for AWS Graviton5 (m9g.metal-48xl) instances.
• #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 documentation.
• #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.

Changed

• #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: 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.

Fixed

• #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: 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: 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: 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: 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: 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, #6041, #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, #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: 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.