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Documentation / efi-stub.txt

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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	=================
2	The EFI Boot Stub
3	=================
5	On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
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 arch/x86/boot/compressed/eboot.c,
11	respectively. For ARM the EFI stub is implemented in
12	arch/arm/boot/compressed/efi-header.S and
13	arch/arm/boot/compressed/efi-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
24	a certain sense it *IS* the boot loader.
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
37	arch/arm/boot/zImage should be copied to the system partition, and it
38	may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
39	should be copied but not necessarily renamed.
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
50	The "initrd=" option
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
56	kernel when it boots.
58	The path to the initrd file must be an absolute path from the
59	beginning of the ESP, relative path names do not work. Also, the path
60	is an EFI-style path and directory elements must be separated with
61	backslashes (\). For example, given the following directory layout::
63	  fs0:>
64		Kernels\
65				bzImage.efi
66				initrd-large.img
68		Ramdisks\
69				initrd-small.img
70				initrd-medium.img
72	to boot with the initrd-large.img file if the current working
73	directory is fs0:\Kernels, the following command must be used::
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,
79	which understands relative paths, whereas the rest of the command line
80	is passed to bzImage.efi.
83	The "dtb=" option
84	-----------------
86	For the ARM and arm64 architectures, we also need to be able to provide a
87	device tree to the kernel. This is done with the "dtb=" command line option,
88	and is processed in the same manner as the "initrd=" option that is
89	described above.
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