Added

• #3837: Added official support for Linux 6.1. See prod-host-setup for some security and performance considerations.
• #4045 and #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: 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.
• Changed default log level from Warn to 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.