1                          The EFI Boot Stub
   2                     ---------------------------
   4On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
   5as a PE/COFF image, thereby convincing EFI firmware loaders to load
   6it as an EFI executable. The code that modifies the bzImage header,
   7along with the EFI-specific entry point that the firmware loader
   8jumps to are collectively known as the "EFI boot stub", and live in
   9arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
  10respectively. For ARM the EFI stub is implemented in
  11arch/arm/boot/compressed/efi-header.S and
  12arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared
  13between architectures is in drivers/firmware/efi/efi-stub-helper.c.
  15For arm64, there is no compressed kernel support, so the Image itself
  16masquerades as a PE/COFF image and the EFI stub is linked into the
  17kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
  18and arch/arm64/kernel/efi-stub.c.
  20By using the EFI boot stub it's possible to boot a Linux kernel
  21without the use of a conventional EFI boot loader, such as grub or
  22elilo. Since the EFI boot stub performs the jobs of a boot loader, in
  23a certain sense it *IS* the boot loader.
  25The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
  28**** How to install bzImage.efi
  30The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
  31System Partition (ESP) and renamed with the extension ".efi". Without
  32the extension the EFI firmware loader will refuse to execute it. It's
  33not possible to execute bzImage.efi from the usual Linux file systems
  34because EFI firmware doesn't have support for them. For ARM the
  35arch/arm/boot/zImage should be copied to the system partition, and it
  36may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
  37should be copied but not necessarily renamed.
  40**** Passing kernel parameters from the EFI shell
  42Arguments to the kernel can be passed after bzImage.efi, e.g.
  44        fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
  47**** The "initrd=" option
  49Like most boot loaders, the EFI stub allows the user to specify
  50multiple initrd files using the "initrd=" option. This is the only EFI
  51stub-specific command line parameter, everything else is passed to the
  52kernel when it boots.
  54The path to the initrd file must be an absolute path from the
  55beginning of the ESP, relative path names do not work. Also, the path
  56is an EFI-style path and directory elements must be separated with
  57backslashes (\). For example, given the following directory layout,
  60        Kernels\
  61                        bzImage.efi
  62                        initrd-large.img
  64        Ramdisks\
  65                        initrd-small.img
  66                        initrd-medium.img
  68to boot with the initrd-large.img file if the current working
  69directory is fs0:\Kernels, the following command must be used,
  71        fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
  73Notice how bzImage.efi can be specified with a relative path. That's
  74because the image we're executing is interpreted by the EFI shell,
  75which understands relative paths, whereas the rest of the command line
  76is passed to bzImage.efi.
  79**** The "dtb=" option
  81For the ARM and arm64 architectures, we also need to be able to provide a
  82device tree to the kernel. This is done with the "dtb=" command line option,
  83and is processed in the same manner as the "initrd=" option that is
  84described above.
  85 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.