Documentation/admin-guide/efi-stub.rst

2 The EFI Boot Stub
6 as a PE/COFF image, thereby convincing EFI firmware loaders to load
7 it as an EFI executable. The code that modifies the bzImage header,
8 along with the EFI-specific entry point that the firmware loader
9 jumps to are collectively known as the "EFI boot stub", and live in
10 arch/x86/boot/header.S and drivers/firmware/efi/libstub/x86-stub.c,
11 respectively. For ARM the EFI stub is implemented in
12 arch/arm/boot/compressed/efi-header.S and
13 drivers/firmware/efi/libstub/arm32-stub.c. EFI stub code that is shared
14 between architectures is in drivers/firmware/efi/libstub.
16 For arm64, there is no compressed kernel support, so the Image itself
17 masquerades as a PE/COFF image and the EFI stub is linked into the
18 kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
19 and drivers/firmware/efi/libstub/arm64-stub.c.
21 By using the EFI boot stub it's possible to boot a Linux kernel
22 without the use of a conventional EFI boot loader, such as grub or
23 elilo. Since the EFI boot stub performs the jobs of a boot loader, in
26 The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
29 How to install bzImage.efi
30 --------------------------
32 The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
33 System Partition (ESP) and renamed with the extension ".efi". Without
34 the extension the EFI firmware loader will refuse to execute it. It's
35 not possible to execute bzImage.efi from the usual Linux file systems
36 because EFI firmware doesn't have support for them. For ARM the
38 may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
42 Passing kernel parameters from the EFI shell
43 --------------------------------------------
45 Arguments to the kernel can be passed after bzImage.efi, e.g.::
47 	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
51 --------------------
53 Like most boot loaders, the EFI stub allows the user to specify
54 multiple initrd files using the "initrd=" option. This is the only EFI
55 stub-specific command line parameter, everything else is passed to the
60 is an EFI-style path and directory elements must be separated with
65 			bzImage.efi
66 			initrd-large.img
69 			initrd-small.img
70 			initrd-medium.img
72 to boot with the initrd-large.img file if the current working
75 	fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
77 Notice how bzImage.efi can be specified with a relative path. That's
78 because the image we're executing is interpreted by the EFI shell,
80 is passed to bzImage.efi.
84 -----------------
86 For the ARM and arm64 architectures, a device tree must be provided to
88 EFI CONFIGURATION TABLE. However, the "dtb=" command line option can
96 as a last resort when a device tree is not provided in the EFI