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Unified Kernel Support Phase 1


Add support for unified kernels images to Fedora.


Current status

Detailed Description

The goal is to move away from initrd images being generated on the installed machine where possible. The initrd is generated while building the kernel package instead, then shipped as part of a unified kernel image (UKI).

A unified kernel image is an all-in-one efi binary containing kernel, initrd, cmdline and signature. The secure boot signature covers everything, specifically the initrd is included which is not the case when the initrd gets loaded as separate file from /boot.

Main motivation for this move is to make the distro more robust and more secure.

Supporting unified kernels for all use cases quickly is not realistic though. Too many features are depending on the current workflow with a host-specific initrd (and host-specific kernel command line), which is fundamentally incompatible with unified kernels where everybody will have the same initrd and command line. Thats why there is 'Phase 1' in title, so we can have more Phases in future releases 😃

long term plan:

  • Phase 1: Get the basic building blocks into place, so it is possible to work with and develop for UKIs in virtual machines.
  • Phase 2+: Expand UKI support, tackling the use cases which depend on a host-specific initrd or command line (see below) one by one.
  • Phase X: Once UKIs have feature parity with non-UKI kernels discuss whenever they should be used by default everywhere (specific use cases like cloud images might switch earlier).
  • NOT planed: remove support for non-UKI kernels.

A host-specific initrd / command line is needed today for:

  • features needing optional dracut modules (initrd rebuild needed to enable them).
  • configuration / secrets baked into the initrd (booting from iscsi for example).
  • configuration being specified on the kernel command line.

systemd has some building blocks which can be used, although none of them are used by fedora today. systemd credentials can be used for secrets (also for configuration). The unified kernel stub can load credentials from the ESP. The unified kernel stub can also load extensions from the ESP, which can possibly be used to replace optional dracut modules.

Phase 1 goals (high priority):

  • Ship a unified kernel image as (optional) kernel sub-rpm. Users can opt-in to use that kernel by installing the sub-rpm. Initial focus is on booting virtual machines where we have a relatively small and well defined set of drivers / features needed. Supporting modern physical machines with standard setup (i.e. boot from local sata/nvme storage) too should be easy.
  • Update kernel install scripts so unified kernels are installed and updated properly.
  • Add bootloader support for unified kernel images. Add unified kernel bls support to grub2, or support using systemd-boot, or both.

Phase 1 goals (lower priority, might move to Phase 2):

  • Add proper discoverable partitions support to installers (anaconda, image builder, ...).
    • Temporary workaround possible: set types using sfdisk in %post script.
    • When using btrfs: configure 'root' subvolume as default volume.
  • Switch cloud images to use unified kernels.

Phase 2/3 goals: moved to Changes/Unified_Kernel_Support_Phase_2

Architecture notes:

  • x86_64 uefi: main target
  • x86_64 bios: experimental grub patches to boot UKIs exist, to allow hybrid uefi/bios cloud images with UKIs.
  • aarch64: depends on sorting out the compressed kernel problem (see systemd issue 23788)
    • kernel got CONFIG_EFI_ZBOOT support in 6.1
    • kernel-ark CI seems to not be ready for it (yet?)
  • ppc64le: uses grub, so building UKIs with a dummy efi stub and letting grub unpack them (simliar to x86_64 bios) should be possible without too much trouble.
    • Not sure this is worth the effort.
    • Probably a good idea to wait for x86_64 experiences before deciding on this one.
    • Some ppc platforms (pseries) have non-EFI secure boot support which should be looked at too.
  • s390x: no plans


Benefit to Fedora

  • Better secure boot support (specifically the initrd is covered by the signature).
  • Better confidential computing support (measurements are much more useful if we know what hashes to expect for the initrd).
  • More robust boot process (generating the initrd on the installed system is fragile, root cause for kernel bugs reported is simply a broken initrd sometimes).


  • Proposal owners:
    • Update kernel build to create unified kernel sub-package.
  • Policies and guidelines: N/A (not needed for this Change)
  • Trademark approval: N/A (not needed for this Change)
  • Alignment with Objectives:

Upgrade/compatibility impact

None (using unified kernel is opt-in for Phase 1).

How To Test

Try on a existing (uefi) system:

  • make sure you are running fedora 37 or rawhide.
  • make sure your root filesystem has type "Linux root (x86-64)" (use fdisk -l to check).
    • should that not be the case use the fdisk tag command ('t') to change the partition type.
  • when using btrfs: make sure the 'root' subvolume is set as default volume.
  • dnf copr enable kraxel/unified.kernel
  • dnf update "grub2*"
  • dnf install kernel-uki-virt
  • reboot

You should find two entries in the grub2 boot menu, one for classing kernel with separate initrd and one for the unified kernel image. Both should boot fine.

The project has kickstart files and helper scripts for generating virtual machine images.

Prebuilt virtual machine images are available from

User Experience


Contingency Plan

  • Contingency mechanism:
    • Probably none (unified kernel images are opt-in for Phase 1).
    • In case we tried switching the cloud images to unified kernels: revert the kickstart config changes.
  • Contingency deadline: N/A (not a System Wide Change)
  • Blocks release? No


N/A (not a System Wide Change)

Release Notes