The EFI Boot Stub
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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.
