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kernel-policy.md

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Firecracker's Kernel Support Policy

Firecracker is tightly coupled with the guest and host kernels on which it is run. This document presents our kernel support policy which aims to help our customers choose host and guest OS configuration, and predict future kernel related changes.

We are continuously validating the currently supported Firecracker releases (as per Firecracker’s release policy) using a combination of:

  • host linux kernel versions 5.10, and 6.1;
  • guest linux kernel versions 4.14, and 5.10.

While other versions and other kernel configs might work, they are not periodically validated in our test suite, and using them might result in unexpected behaviour. Starting with release v1.0 each major and minor release will specify the supported kernel versions.

Once a kernel version is officially enabled, it is supported for a minimum of 2 years. Adding support for a new kernel version will result in a Firecracker release only if compatibility changes are required.

Host kernel Guest kernel v4.14 Guest kernel v5.10 Min. end of support
v5.10 Y Y 2024-01-31
v6.1 Y Y 2025-10-12

The guest kernel configs used in our validation pipelines can be found here while a breakdown of the relevant guest kernel modules can be found in the next section.

Guest kernel configuration items

The configuration items that may be relevant for Firecracker are:

  • serial console - CONFIG_SERIAL_8250_CONSOLE, CONFIG_PRINTK
  • initrd support - CONFIG_BLK_DEV_INITRD
  • virtio devices - CONFIG_VIRTIO_MMIO
    • balloon - CONFIG_MEMORY_BALLOON, CONFIG_VIRTIO_BALLOON
    • block - CONFIG_VIRTIO_BLK
      • partuuid support - CONFIG_MSDOS_PARTITION
    • network - CONFIG_VIRTIO_NET
    • vsock - CONFIG_VIRTIO_VSOCKETS
    • entropy - CONFIG_HW_RANDOM_VIRTIO
  • guest RNG - CONFIG_RANDOM_TRUST_CPU
    • use CPU RNG instructions (if present) to initialize RNG. Available for >= 5.10
  • ACPI support - CONFIG_ACPI and CONFIG_PCI

There are also guest config options which are dependant on the platform on which Firecracker is run:

ARM

  • timekeeping - CONFIG_ARM_AMBA, CONFIG_RTC_DRV_PL031
  • serial console - CONFIG_SERIAL_OF_PLATFORM

x86_64

  • timekeeping - CONFIG_KVM_GUEST (which enables CONFIG_KVM_CLOCK)
  • high precision timekeeping - CONFIG_PTP_1588_CLOCK, CONFIG_PTP_1588_CLOCK_KVM
  • external clean shutdown - CONFIG_SERIO_I8042, CONFIG_KEYBOARD_ATKBD
  • virtio devices - CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES

Minimal boot requirements

Depending on the source of boot (either from a block device or from an initrd), the minimal configuration for a guest kernel for a successful microVM boot is:

  • Booting with initrd:

    • CONFIG_BLK_DEV_INITRD=y
      • aarch64 CONFIG_VIRTIO_MMIO=y (for the serial device).
      • x86_64 CONFIG_KVM_GUEST=y.

  • Booting with root block device:

    • aarch64
      • CONFIG_VIRTIO_BLK=y
    • x86_64
      • CONFIG_VIRTIO_BLK=y
      • CONFIG_ACPI=y
      • CONFIG_PCI=y
      • CONFIG_KVM_GUEST=y.

Optional: To enable boot logs set CONFIG_SERIAL_8250_CONSOLE=y and CONFIG_PRINTK=y in the guest kernel config.

Booting with ACPI (x86_64 only):

Firecracker supports booting kernels with ACPI support. The relevant configurations for the guest kernel are:

  • CONFIG_ACPI=y
  • CONFIG_PCI=y

Please note that Firecracker does not support PCI devices. The CONFIG_PCI option is needed for ACPI initialization inside the guest.

ACPI supersedes the legacy way of booting a microVM, i.e. via MPTable and command line parameters for VirtIO devices.

We suggest that users disable MPTable and passing VirtIO devices via kernel command line parameters. These boot mechanisms are now deprecated. Users can disable these features by disabling the corresponding guest kernel configuration parameters:

  • CONFIG_X86_MPPARSE=n
  • CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=n

During the deprecation period Firecracker will continue to support the legacy way of booting a microVM. Firecracker will be able to boot kernels with the following configurations:

  • Only ACPI
  • Only legacy mechanisms
  • Both ACPI and legacy mechanisms

Caveats

  • Snapshot compatibility across kernel versions
  • When booting with kernels that support both ACPI and legacy boot mechanisms Firecracker passes VirtIO devices to the guest twice, once through ACPI and a second time via kernel command line parameters. In these cases, the guest tries to initialize devices twice. The second time, initialization fails and the guest will emit warning messages in dmesg, however the devices will work correctly.