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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,
-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,
- 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.