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Commit | Line | Data |
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1 | ================= | |
2 | The EFI Boot Stub | |
3 | ================= | |
4 | ||
5 | On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade | |
6 | as a PE/COFF image, thereby convincing EFI firmware loaders to load | |
7 | it as an EFI executable. The code that modifies the bzImage header, | |
8 | along with the EFI-specific entry point that the firmware loader | |
9 | jumps to are collectively known as the "EFI boot stub", and live in | |
10 | arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c, | |
11 | respectively. For ARM the EFI stub is implemented in | |
12 | arch/arm/boot/compressed/efi-header.S and | |
13 | arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared | |
14 | between architectures is in drivers/firmware/efi/libstub. | |
15 | ||
16 | For arm64, there is no compressed kernel support, so the Image itself | |
17 | masquerades as a PE/COFF image and the EFI stub is linked into the | |
18 | kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S | |
19 | and drivers/firmware/efi/libstub/arm64-stub.c. | |
20 | ||
21 | By using the EFI boot stub it's possible to boot a Linux kernel | |
22 | without the use of a conventional EFI boot loader, such as grub or | |
23 | elilo. Since the EFI boot stub performs the jobs of a boot loader, in | |
24 | a certain sense it *IS* the boot loader. | |
25 | ||
26 | The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. | |
27 | ||
28 | ||
29 | How to install bzImage.efi | |
30 | -------------------------- | |
31 | ||
32 | The bzImage located in arch/x86/boot/bzImage must be copied to the EFI | |
33 | System Partition (ESP) and renamed with the extension ".efi". Without | |
34 | the extension the EFI firmware loader will refuse to execute it. It's | |
35 | not possible to execute bzImage.efi from the usual Linux file systems | |
36 | because EFI firmware doesn't have support for them. For ARM the | |
37 | arch/arm/boot/zImage should be copied to the system partition, and it | |
38 | may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image | |
39 | should be copied but not necessarily renamed. | |
40 | ||
41 | ||
42 | Passing kernel parameters from the EFI shell | |
43 | -------------------------------------------- | |
44 | ||
45 | Arguments to the kernel can be passed after bzImage.efi, e.g.:: | |
46 | ||
47 | fs0:> bzImage.efi console=ttyS0 root=/dev/sda4 | |
48 | ||
49 | ||
50 | The "initrd=" option | |
51 | -------------------- | |
52 | ||
53 | Like most boot loaders, the EFI stub allows the user to specify | |
54 | multiple initrd files using the "initrd=" option. This is the only EFI | |
55 | stub-specific command line parameter, everything else is passed to the | |
56 | kernel when it boots. | |
57 | ||
58 | The path to the initrd file must be an absolute path from the | |
59 | beginning of the ESP, relative path names do not work. Also, the path | |
60 | is an EFI-style path and directory elements must be separated with | |
61 | backslashes (\). For example, given the following directory layout:: | |
62 | ||
63 | fs0:> | |
64 | Kernels\ | |
65 | bzImage.efi | |
66 | initrd-large.img | |
67 | ||
68 | Ramdisks\ | |
69 | initrd-small.img | |
70 | initrd-medium.img | |
71 | ||
72 | to boot with the initrd-large.img file if the current working | |
73 | directory is fs0:\Kernels, the following command must be used:: | |
74 | ||
75 | fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img | |
76 | ||
77 | Notice how bzImage.efi can be specified with a relative path. That's | |
78 | because the image we're executing is interpreted by the EFI shell, | |
79 | which understands relative paths, whereas the rest of the command line | |
80 | is passed to bzImage.efi. | |
81 | ||
82 | ||
83 | The "dtb=" option | |
84 | ----------------- | |
85 | ||
86 | For the ARM and arm64 architectures, we also need to be able to provide a | |
87 | device tree to the kernel. This is done with the "dtb=" command line option, | |
88 | and is processed in the same manner as the "initrd=" option that is | |
89 | described above. |