efi-stub.txt 3.2 KB

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