EFI stub kernel

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This article has some todo items:
  • CONFIG_PM_STD_PARTITION for hibernation

The (U)EFI firmware present in many computers can function as bootloader, allowing systems to boot without needing the addition of a secondary bootloader. This article provides instructions on configuring and installing unsigned kernels in the EFI System Partition (ESP) of a computer running in EFI mode. This requires secure boot to be disabled.

Kernel configuration

In order to boot directly from UEFI, the kernel needs to know where to find the root (/) partition of the system to be booted. Enable

  • EFI runtime service support (CONFIG_EFI),
  • EFI stub support (CONFIG_EFI_STUB)
  • Built-in kernel command line (CONFIG_CMDLINE_BOOL)
  • and add the root partition path (example: /dev/sda2) or its PARTUUID to (CONFIG_CMDLINE).
KERNEL Enable EFI stub support
Processor type and features  --->
    [*] EFI runtime service support 
    [*]   EFI stub support
    [ ]     EFI mixed-mode support
    [*] Built-in kernel command line

Using root=PARTUUID= might be preferable. To find out use blkid:

The partition's PARTUUID is distinct from the filesystem's UUID.
root #blkid | grep sda2
/dev/sda2: UUID="d1e0c1e0-3a40-42c5-8931-cfa2c7deae32" TYPE="ext4" PARTUUID="adf55784-15d9-4ca3-bb3f-56de0b35d88d"
KERNEL CONFIG_CMDLINE - alternative using PARTUUID (example: adf55784-15d9-4ca3-bb3f-56de0b35d88d)
Processor type and features  --->
    [*] Built-in kernel command line
Alternatively to setting the Built-in kernel command line, users of a distribution kernel can have the root partition path be set by the NVRAM (boot entry) (e.g. via efibootmgr) or by an additional EFI boot manager.


If an ESP does not exist, one needs to be created. See EFI System Partition. Still in the kernel directory, build the kernel and install the modules:

root #cd /usr/src/linux
root #make && make modules_install

Have the ESP (in this example /dev/sda1) mounted at /boot:

root #mount /dev/sda1 /boot

Copy or move the kernel image to the right place adding version number and the .efi suffix:

root #mkdir -p /boot/EFI/Gentoo
root #cp arch/x86/boot/bzImage /boot/EFI/Gentoo/bzImage-4.9.76-r1.efi

In the ESP it should then be listed like:

user $tree -L 3 /boot
 └── EFI
     └── Gentoo
         └── bzImage-4.9.76-r1.efi

Alternatively the fallback directory /boot/EFI/Boot could be used additionally to or instead of /boot/EFI/Gentoo.

Next, update the NVRAM creating a new boot entry using e.g. efibootmgr.


For example:

root #efibootmgr -c -L "EFI Stub" -l '\EFI\Gentoo\bzImage-4.9.76-r1.efi'
Some firmware delete boot entries if the associated device is not found. If need to reconnect associated device between reboots be sure there is the way to re-add boot entries for it, from emergency boot media for example.

Microcode loading (Intel)

An efistub compatible method to load microcode without using an initramfs can be found in the Intel microcode article.

Optional: Initramfs

If for some reason an initramfs is needed, it can either be embedded into the kernel or be used as a separate file.

In case it's desired as a separate file, it should also be copied or moved into the ESP and the NVRAM be updated accordingly.

root #mv /boot/initramfs-4.9.76-r1-gentoo.img /boot/EFI/Gentoo/initramfs.img
root #mount /sys/firmware/efi/efivars -o rw,remount
root #efibootmgr --create --part 1 --disk /dev/sda --label "Gentoo" --loader '\efi\gentoo\bzImage-4.9.76-r1.efi' -u 'initrd=\efi\gentoo\initramfs.img'
root #mount /sys/firmware/efi/efivars -o ro,remount

Some UEFI implementations however seem to not support passing parameters from the NVRAM to the EFI stub kernel.

For embedding the initramfs directly into the kernel image, the Initramfs source file(s) (CONFIG_INITRAMFS_SOURCE) must be coded in the kernel (directly under the Initial RAM filesystem and RAM disk (initramfs/initrd) support (CONFIG_BLK_DEV_INITRD) option) as shown here for a custom initramfs created in /usr/src/initramfs:

KERNEL CONFIG_INITRAMFS_SOURCE="/usr/src/initramfs" — (Example from Custom Initramfs)
General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
    (/usr/src/initramfs) Initramfs source file(s)

Embedding takes place on compilation, so the kernel must then be recompiled and be moved (or copied) again into the ESP.

Optional: Secure boot

There is a forums post and another wiki article explaining in detail how to sign the kernel and enable secure boot.


See also

External resources