In the Fedora Linux 38 release next year, the Fedora Project considers switching from creating fresh initrd images on computers performing upgrades to using unified kernel images (UKI).

Changes/Unified Kernel Support Phase 1 - Fedora Project Wiki

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

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.

Switching the whole distro over to unified kernels 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

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.
  • root filesystem being the most important one.  Discoverable partitions allow to remove this.

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.
• Add proper systemd-boot support to installers.
  • Temporary workaround possible: run 'bootctl install' in %post script.
• Better measurement and remote attestation support.
  • store kernel + initrd hashes somewhere (kernel-hashes.rpm ?) to allow pre-calculate TPM PCR values.
  • avoid using grub2 (measures every config file line executed which is next to impossible to pre-calculate).
• Switch cloud images to use unified kernels.

Phase 2/3 goals (longer-term stuff which is not realistic to complete for F38).

• Move away from using the kernel command line for configuration.
• Move away from storing secrets in the initrd.
• Handle dracut optional modules in a different way.

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.

Changes/Unified Kernel Support Phase 1 - Fedora Project Wiki