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efi-stub.txt

efi-stub.txt 2.3 KB
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  1. 			  The EFI Boot Stub
    2. 

  2. 		     ---------------------------
    3. 

  3. 

  4. On the x86 platform, a bzImage can masquerade as a PE/COFF image,
    5. 

  5. thereby convincing EFI firmware loaders to load it as an EFI
    6. 

  6. executable. The code that modifies the bzImage header, along with the
    7. 

  7. EFI-specific entry point that the firmware loader jumps to are
    8. 

  8. collectively known as the "EFI boot stub", and live in
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  9. arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
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  10. respectively.
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  11. 

  12. By using the EFI boot stub it's possible to boot a Linux kernel
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  13. without the use of a conventional EFI boot loader, such as grub or
    14. 

  14. elilo. Since the EFI boot stub performs the jobs of a boot loader, in
    15. 

  15. a certain sense it *IS* the boot loader.
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  16. 

  17. The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
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  18. 

  19. 

  20. **** How to install bzImage.efi
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  21. 

  22. The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
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  23. System Partiion (ESP) and renamed with the extension ".efi". Without
    24. 

  24. the extension the EFI firmware loader will refuse to execute it. It's
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  25. not possible to execute bzImage.efi from the usual Linux file systems
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  26. because EFI firmware doesn't have support for them.
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  27. 

  28. 

  29. **** Passing kernel parameters from the EFI shell
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  30. 

  31. Arguments to the kernel can be passed after bzImage.efi, e.g.
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  32. 

  33. 	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
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  34. 

  35. 

  36. **** The "initrd=" option
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  37. 

  38. Like most boot loaders, the EFI stub allows the user to specify
    39. 

  39. multiple initrd files using the "initrd=" option. This is the only EFI
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  40. stub-specific command line parameter, everything else is passed to the
    41. 

  41. kernel when it boots.
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  42. 

  43. The path to the initrd file must be an absolute path from the
    44. 

  44. beginning of the ESP, relative path names do not work. Also, the path
    45. 

  45. is an EFI-style path and directory elements must be separated with
    46. 

  46. backslashes (\). For example, given the following directory layout,
    47. 

  47. 

  48. fs0:>
    49. 

  49. 	Kernels\
    50. 

  50. 			bzImage.efi
    51. 

  51. 			initrd-large.img
    52. 

  52. 

  53. 	Ramdisks\
    54. 

  54. 			initrd-small.img
    55. 

  55. 			initrd-medium.img
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  56. 

  57. to boot with the initrd-large.img file if the current working
    58. 

  58. directory is fs0:\Kernels, the following command must be used,
    59. 

  59. 

  60. 	fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
    61. 

  61. 

  62. Notice how bzImage.efi can be specified with a relative path. That's
    63. 

  63. because the image we're executing is interpreted by the EFI shell,
    64. 

  64. which understands relative paths, whereas the rest of the command line
    65. 

  65. is passed to bzImage.efi.
    66.