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 an additional software bootloader. This article shows how to configure and install an unsigned kernel in the EFI System Partition (ESP) of a computer running in EFI mode with secureboot turned off.

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) and 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
    (root=/dev/sda2)

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

Note
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
    (root=PARTUUID=adf55784-15d9-4ca3-bb3f-56de0b35d88d)

It is also a good idea to enable EFI Variable Support via sysfs (CONFIG_EFI_VARS) so that the efivars can be mounted. CONFIG_EFI_MIXED as proposed by Gentoo Handbook is not needed for the EFI boot stub.

KERNEL EFI Variable Support via sysfs
Firmware Drivers  --->
    EFI (Extensible Firmware Interface) Support  --->
        <*> EFI Variable Support via sysfs

Installation

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-4.9.16.efi

In the ESP it should then be listed like:

user $tree -L 3 /boot
 /boot
 └── EFI
     └── Gentoo
         └── bzImage-4.9.16.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.

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-4.9.16-gentoo.img /boot/EFI/Gentoo/initramfs.img
root #mount /sys/firmware/efi/efivars -o rw,remount
root #efibootmgr --create --part 1 --label "Gentoo" --loader '\efi\gentoo\bzImage-4.9.16.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.

Note
This guide was created with the x64 architecture in mind. According the specification, UEFI is rich enough to cover a range of contemporary processor designs.
UEFI image's name is composed by the machine type defined in the PE32+ format [1] [2]. For example, a UEFI OS loader for the x86 architecture is bootIA32.efi

See also

External resources

References