| Kyle Swenson | 8d8f654 | 2021-03-15 11:02:55 -0600 | [diff] [blame] | 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/efi-stub-helper.c. |
| 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 arch/arm64/kernel/efi-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. |