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Linux/Documentation/efi-stub.txt

  1                           The EFI Boot Stub
  2                      ---------------------------
  3 
  4 On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
  5 as a PE/COFF image, thereby convincing EFI firmware loaders to load
  6 it as an EFI executable. The code that modifies the bzImage header,
  7 along with the EFI-specific entry point that the firmware loader
  8 jumps to are collectively known as the "EFI boot stub", and live in
  9 arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
 10 respectively. For ARM the EFI stub is implemented in
 11 arch/arm/boot/compressed/efi-header.S and
 12 arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared
 13 between architectures is in drivers/firmware/efi/libstub.
 14 
 15 For arm64, there is no compressed kernel support, so the Image itself
 16 masquerades as a PE/COFF image and the EFI stub is linked into the
 17 kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
 18 and drivers/firmware/efi/libstub/arm64-stub.c.
 19 
 20 By using the EFI boot stub it's possible to boot a Linux kernel
 21 without the use of a conventional EFI boot loader, such as grub or
 22 elilo. Since the EFI boot stub performs the jobs of a boot loader, in
 23 a certain sense it *IS* the boot loader.
 24 
 25 The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
 26 
 27 
 28 **** How to install bzImage.efi
 29 
 30 The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
 31 System Partition (ESP) and renamed with the extension ".efi". Without
 32 the extension the EFI firmware loader will refuse to execute it. It's
 33 not possible to execute bzImage.efi from the usual Linux file systems
 34 because EFI firmware doesn't have support for them. For ARM the
 35 arch/arm/boot/zImage should be copied to the system partition, and it
 36 may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
 37 should be copied but not necessarily renamed.
 38 
 39 
 40 **** Passing kernel parameters from the EFI shell
 41 
 42 Arguments to the kernel can be passed after bzImage.efi, e.g.
 43 
 44         fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
 45 
 46 
 47 **** The "initrd=" option
 48 
 49 Like most boot loaders, the EFI stub allows the user to specify
 50 multiple initrd files using the "initrd=" option. This is the only EFI
 51 stub-specific command line parameter, everything else is passed to the
 52 kernel when it boots.
 53 
 54 The path to the initrd file must be an absolute path from the
 55 beginning of the ESP, relative path names do not work. Also, the path
 56 is an EFI-style path and directory elements must be separated with
 57 backslashes (\). For example, given the following directory layout,
 58 
 59 fs0:>
 60         Kernels\
 61                         bzImage.efi
 62                         initrd-large.img
 63 
 64         Ramdisks\
 65                         initrd-small.img
 66                         initrd-medium.img
 67 
 68 to boot with the initrd-large.img file if the current working
 69 directory is fs0:\Kernels, the following command must be used,
 70 
 71         fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
 72 
 73 Notice how bzImage.efi can be specified with a relative path. That's
 74 because the image we're executing is interpreted by the EFI shell,
 75 which understands relative paths, whereas the rest of the command line
 76 is passed to bzImage.efi.
 77 
 78 
 79 **** The "dtb=" option
 80 
 81 For the ARM and arm64 architectures, we also need to be able to provide a
 82 device tree to the kernel. This is done with the "dtb=" command line option,
 83 and is processed in the same manner as the "initrd=" option that is
 84 described above.

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