projects / linux-2.6-block.git / blame
| Commit | Line | Data |
|---|---|---|
| 0c759662 MF |
1 | The EFI Boot Stub |
| 2 | --------------------------- | |
| 3 | ||
| 719e2843 RF |
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 | |
| 0c759662 | 9 | arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c, |
| 719e2843 RF |
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. | |
| 0c759662 MF |
14 | |
| 15 | By using the EFI boot stub it's possible to boot a Linux kernel | |
| 16 | without the use of a conventional EFI boot loader, such as grub or | |
| 17 | elilo. Since the EFI boot stub performs the jobs of a boot loader, in | |
| 18 | a certain sense it *IS* the boot loader. | |
| 19 | ||
| 20 | The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. | |
| 21 | ||
| 22 | ||
| 23 | **** How to install bzImage.efi | |
| 24 | ||
| 25 | The bzImage located in arch/x86/boot/bzImage must be copied to the EFI | |
| bf651883 | 26 | System Partition (ESP) and renamed with the extension ".efi". Without |
| 0c759662 MF |
27 | the extension the EFI firmware loader will refuse to execute it. It's |
| 28 | not possible to execute bzImage.efi from the usual Linux file systems | |
| 719e2843 RF |
29 | because EFI firmware doesn't have support for them. For ARM the |
| 30 | arch/arm/boot/zImage should be copied to the system partition, and it | |
| 31 | may not need to be renamed. | |
| 0c759662 MF |
32 | |
| 33 | ||
| 34 | **** Passing kernel parameters from the EFI shell | |
| 35 | ||
| 36 | Arguments to the kernel can be passed after bzImage.efi, e.g. | |
| 37 | ||
| 38 | fs0:> bzImage.efi console=ttyS0 root=/dev/sda4 | |
| 39 | ||
| 40 | ||
| 41 | **** The "initrd=" option | |
| 42 | ||
| 43 | Like most boot loaders, the EFI stub allows the user to specify | |
| 44 | multiple initrd files using the "initrd=" option. This is the only EFI | |
| 45 | stub-specific command line parameter, everything else is passed to the | |
| 46 | kernel when it boots. | |
| 47 | ||
| 48 | The path to the initrd file must be an absolute path from the | |
| 49 | beginning of the ESP, relative path names do not work. Also, the path | |
| 50 | is an EFI-style path and directory elements must be separated with | |
| 51 | backslashes (\). For example, given the following directory layout, | |
| 52 | ||
| 53 | fs0:> | |
| 54 | Kernels\ | |
| 55 | bzImage.efi | |
| 56 | initrd-large.img | |
| 57 | ||
| 58 | Ramdisks\ | |
| 59 | initrd-small.img | |
| 60 | initrd-medium.img | |
| 61 | ||
| 62 | to boot with the initrd-large.img file if the current working | |
| 63 | directory is fs0:\Kernels, the following command must be used, | |
| 64 | ||
| 65 | fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img | |
| 66 | ||
| 67 | Notice how bzImage.efi can be specified with a relative path. That's | |
| 68 | because the image we're executing is interpreted by the EFI shell, | |
| 69 | which understands relative paths, whereas the rest of the command line | |
| 70 | is passed to bzImage.efi. | |
| 719e2843 RF |
71 | |
| 72 | ||
| 73 | **** The "dtb=" option | |
| 74 | ||
| 75 | For the ARM architecture, we also need to be able to provide a device | |
| 76 | tree to the kernel. This is done with the "dtb=" command line option, | |
| 77 | and is processed in the same manner as the "initrd=" option that is | |
| 78 | described above. |