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September 2, 2014 08:03
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The EFI Boot Stub
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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. |
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