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Documentation / efi-stub.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 EST.

1				  The EFI Boot Stub
2			     ---------------------------
3	
4	On the x86 platform, a bzImage can masquerade as a PE/COFF image,
5	thereby convincing EFI firmware loaders to load it as an EFI
6	executable. The code that modifies the bzImage header, along with the
7	EFI-specific entry point that the firmware loader jumps to are
8	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.
11	
12	By using the EFI boot stub it's possible to boot a Linux kernel
13	without the use of a conventional EFI boot loader, such as grub or
14	elilo. Since the EFI boot stub performs the jobs of a boot loader, in
15	a certain sense it *IS* the boot loader.
16	
17	The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
18	
19	
20	**** How to install bzImage.efi
21	
22	The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
23	System Partition (ESP) and renamed with the extension ".efi". Without
24	the extension the EFI firmware loader will refuse to execute it. It's
25	not possible to execute bzImage.efi from the usual Linux file systems
26	because EFI firmware doesn't have support for them.
27	
28	
29	**** Passing kernel parameters from the EFI shell
30	
31	Arguments to the kernel can be passed after bzImage.efi, e.g.
32	
33		fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
34	
35	
36	**** The "initrd=" option
37	
38	Like most boot loaders, the EFI stub allows the user to specify
39	multiple initrd files using the "initrd=" option. This is the only EFI
40	stub-specific command line parameter, everything else is passed to the
41	kernel when it boots.
42	
43	The path to the initrd file must be an absolute path from the
44	beginning of the ESP, relative path names do not work. Also, the path
45	is an EFI-style path and directory elements must be separated with
46	backslashes (\). For example, given the following directory layout,
47	
48	fs0:>
49		Kernels\
50				bzImage.efi
51				initrd-large.img
52	
53		Ramdisks\
54				initrd-small.img
55				initrd-medium.img
56	
57	to boot with the initrd-large.img file if the current working
58	directory is fs0:\Kernels, the following command must be used,
59	
60		fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
61	
62	Notice how bzImage.efi can be specified with a relative path. That's
63	because the image we're executing is interpreted by the EFI shell,
64	which understands relative paths, whereas the rest of the command line
65	is passed to bzImage.efi.
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