linux/Documentation/x86/efi-stub.txt
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   1                          The EFI Boot Stub
   2                     ---------------------------
   3
   4On the x86 platform, a bzImage can masquerade as a PE/COFF image,
   5thereby convincing EFI firmware loaders to load it as an EFI
   6executable. The code that modifies the bzImage header, along with the
   7EFI-specific entry point that the firmware loader jumps to are
   8collectively known as the "EFI boot stub", and live in
   9arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
  10respectively.
  11
  12By using the EFI boot stub it's possible to boot a Linux kernel
  13without the use of a conventional EFI boot loader, such as grub or
  14elilo. Since the EFI boot stub performs the jobs of a boot loader, in
  15a certain sense it *IS* the boot loader.
  16
  17The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
  18
  19
  20**** How to install bzImage.efi
  21
  22The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
  23System Partiion (ESP) and renamed with the extension ".efi". Without
  24the extension the EFI firmware loader will refuse to execute it. It's
  25not possible to execute bzImage.efi from the usual Linux file systems
  26because EFI firmware doesn't have support for them.
  27
  28
  29**** Passing kernel parameters from the EFI shell
  30
  31Arguments 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
  38Like most boot loaders, the EFI stub allows the user to specify
  39multiple initrd files using the "initrd=" option. This is the only EFI
  40stub-specific command line parameter, everything else is passed to the
  41kernel when it boots.
  42
  43The path to the initrd file must be an absolute path from the
  44beginning of the ESP, relative path names do not work. Also, the path
  45is an EFI-style path and directory elements must be separated with
  46backslashes (\). For example, given the following directory layout,
  47
  48fs0:>
  49        Kernels\
  50                        bzImage.efi
  51                        initrd-large.img
  52
  53        Ramdisks\
  54                        initrd-small.img
  55                        initrd-medium.img
  56
  57to boot with the initrd-large.img file if the current working
  58directory is fs0:\Kernels, the following command must be used,
  59
  60        fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
  61
  62Notice how bzImage.efi can be specified with a relative path. That's
  63because the image we're executing is interpreted by the EFI shell,
  64which understands relative paths, whereas the rest of the command line
  65is passed to bzImage.efi.
  66
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