mirror of
https://github.com/torvalds/linux.git
synced 2024-11-14 16:12:02 +00:00
0c7596621e
Since we can't expect every user to read the EFI boot stub code it seems prudent to have a couple of paragraphs explaining what it is and how it works. The "initrd=" option in particular is tricky because it only understands absolute EFI-style paths (backslashes as directory separators), and until now this hasn't been documented anywhere. This has tripped up a couple of users. Cc: Matthew Garrett <mjg@redhat.com> Cc: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Matt Fleming <matt.fleming@intel.com> Link: http://lkml.kernel.org/r/1331907517-3985-4-git-send-email-matt@console-pimps.org Signed-off-by: H. Peter Anvin <hpa@zytor.com>
66 lines
2.3 KiB
Plaintext
66 lines
2.3 KiB
Plaintext
The EFI Boot Stub
|
|
---------------------------
|
|
|
|
On the x86 platform, a bzImage can masquerade as a PE/COFF image,
|
|
thereby convincing EFI firmware loaders to load it as an EFI
|
|
executable. The code that modifies the bzImage header, along with the
|
|
EFI-specific entry point that the firmware loader jumps to are
|
|
collectively known as the "EFI boot stub", and live in
|
|
arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
|
|
respectively.
|
|
|
|
By using the EFI boot stub it's possible to boot a Linux kernel
|
|
without the use of a conventional EFI boot loader, such as grub or
|
|
elilo. Since the EFI boot stub performs the jobs of a boot loader, in
|
|
a certain sense it *IS* the boot loader.
|
|
|
|
The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
|
|
|
|
|
|
**** How to install bzImage.efi
|
|
|
|
The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
|
|
System Partiion (ESP) and renamed with the extension ".efi". Without
|
|
the extension the EFI firmware loader will refuse to execute it. It's
|
|
not possible to execute bzImage.efi from the usual Linux file systems
|
|
because EFI firmware doesn't have support for them.
|
|
|
|
|
|
**** Passing kernel parameters from the EFI shell
|
|
|
|
Arguments to the kernel can be passed after bzImage.efi, e.g.
|
|
|
|
fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
|
|
|
|
|
|
**** The "initrd=" option
|
|
|
|
Like most boot loaders, the EFI stub allows the user to specify
|
|
multiple initrd files using the "initrd=" option. This is the only EFI
|
|
stub-specific command line parameter, everything else is passed to the
|
|
kernel when it boots.
|
|
|
|
The path to the initrd file must be an absolute path from the
|
|
beginning of the ESP, relative path names do not work. Also, the path
|
|
is an EFI-style path and directory elements must be separated with
|
|
backslashes (\). For example, given the following directory layout,
|
|
|
|
fs0:>
|
|
Kernels\
|
|
bzImage.efi
|
|
initrd-large.img
|
|
|
|
Ramdisks\
|
|
initrd-small.img
|
|
initrd-medium.img
|
|
|
|
to boot with the initrd-large.img file if the current working
|
|
directory is fs0:\Kernels, the following command must be used,
|
|
|
|
fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
|
|
|
|
Notice how bzImage.efi can be specified with a relative path. That's
|
|
because the image we're executing is interpreted by the EFI shell,
|
|
which understands relative paths, whereas the rest of the command line
|
|
is passed to bzImage.efi.
|