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Documentation / efi-stub.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

1				  The EFI Boot Stub
2			     ---------------------------
3	
4	On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
5	as a PE/COFF image, thereby convincing EFI firmware loaders to load
6	it as an EFI executable. The code that modifies the bzImage header,
7	along with the EFI-specific entry point that the firmware loader
8	jumps to are collectively known as the "EFI boot stub", and live in
9	arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
10	respectively. For ARM the EFI stub is implemented in
11	arch/arm/boot/compressed/efi-header.S and
12	arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared
13	between architectures is in drivers/firmware/efi/efi-stub-helper.c.
14	
15	For arm64, there is no compressed kernel support, so the Image itself
16	masquerades as a PE/COFF image and the EFI stub is linked into the
17	kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
18	and arch/arm64/kernel/efi-stub.c.
19	
20	By using the EFI boot stub it's possible to boot a Linux kernel
21	without the use of a conventional EFI boot loader, such as grub or
22	elilo. Since the EFI boot stub performs the jobs of a boot loader, in
23	a certain sense it *IS* the boot loader.
24	
25	The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
26	
27	
28	**** How to install bzImage.efi
29	
30	The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
31	System Partition (ESP) and renamed with the extension ".efi". Without
32	the extension the EFI firmware loader will refuse to execute it. It's
33	not possible to execute bzImage.efi from the usual Linux file systems
34	because EFI firmware doesn't have support for them. For ARM the
35	arch/arm/boot/zImage should be copied to the system partition, and it
36	may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
37	should be copied but not necessarily renamed.
38	
39	
40	**** Passing kernel parameters from the EFI shell
41	
42	Arguments to the kernel can be passed after bzImage.efi, e.g.
43	
44		fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
45	
46	
47	**** The "initrd=" option
48	
49	Like most boot loaders, the EFI stub allows the user to specify
50	multiple initrd files using the "initrd=" option. This is the only EFI
51	stub-specific command line parameter, everything else is passed to the
52	kernel when it boots.
53	
54	The path to the initrd file must be an absolute path from the
55	beginning of the ESP, relative path names do not work. Also, the path
56	is an EFI-style path and directory elements must be separated with
57	backslashes (\). For example, given the following directory layout,
58	
59	fs0:>
60		Kernels\
61				bzImage.efi
62				initrd-large.img
63	
64		Ramdisks\
65				initrd-small.img
66				initrd-medium.img
67	
68	to boot with the initrd-large.img file if the current working
69	directory is fs0:\Kernels, the following command must be used,
70	
71		fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
72	
73	Notice how bzImage.efi can be specified with a relative path. That's
74	because the image we're executing is interpreted by the EFI shell,
75	which understands relative paths, whereas the rest of the command line
76	is passed to bzImage.efi.
77	
78	
79	**** The "dtb=" option
80	
81	For the ARM and arm64 architectures, we also need to be able to provide a
82	device tree to the kernel. This is done with the "dtb=" command line option,
83	and is processed in the same manner as the "initrd=" option that is
84	described above.
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