1 ================= 1 =================
2 The EFI Boot Stub 2 The EFI Boot Stub
3 ================= 3 =================
4 4
5 On the x86 and ARM platforms, a kernel zImage/ 5 On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
6 as a PE/COFF image, thereby convincing EFI fir 6 as a PE/COFF image, thereby convincing EFI firmware loaders to load
7 it as an EFI executable. The code that modifie 7 it as an EFI executable. The code that modifies the bzImage header,
8 along with the EFI-specific entry point that t 8 along with the EFI-specific entry point that the firmware loader
9 jumps to are collectively known as the "EFI bo 9 jumps to are collectively known as the "EFI boot stub", and live in
10 arch/x86/boot/header.S and drivers/firmware/ef 10 arch/x86/boot/header.S and drivers/firmware/efi/libstub/x86-stub.c,
11 respectively. For ARM the EFI stub is implemen 11 respectively. For ARM the EFI stub is implemented in
12 arch/arm/boot/compressed/efi-header.S and 12 arch/arm/boot/compressed/efi-header.S and
13 drivers/firmware/efi/libstub/arm32-stub.c. EFI 13 drivers/firmware/efi/libstub/arm32-stub.c. EFI stub code that is shared
14 between architectures is in drivers/firmware/e 14 between architectures is in drivers/firmware/efi/libstub.
15 15
16 For arm64, there is no compressed kernel suppo 16 For arm64, there is no compressed kernel support, so the Image itself
17 masquerades as a PE/COFF image and the EFI stu 17 masquerades as a PE/COFF image and the EFI stub is linked into the
18 kernel. The arm64 EFI stub lives in drivers/fi 18 kernel. The arm64 EFI stub lives in drivers/firmware/efi/libstub/arm64.c
19 and drivers/firmware/efi/libstub/arm64-stub.c. 19 and drivers/firmware/efi/libstub/arm64-stub.c.
20 20
21 By using the EFI boot stub it's possible to bo 21 By using the EFI boot stub it's possible to boot a Linux kernel
22 without the use of a conventional EFI boot loa 22 without the use of a conventional EFI boot loader, such as grub or
23 elilo. Since the EFI boot stub performs the jo 23 elilo. Since the EFI boot stub performs the jobs of a boot loader, in
24 a certain sense it *IS* the boot loader. 24 a certain sense it *IS* the boot loader.
25 25
26 The EFI boot stub is enabled with the CONFIG_E 26 The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
27 27
28 28
29 How to install bzImage.efi 29 How to install bzImage.efi
30 -------------------------- 30 --------------------------
31 31
32 The bzImage located in arch/x86/boot/bzImage m 32 The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
33 System Partition (ESP) and renamed with the ex 33 System Partition (ESP) and renamed with the extension ".efi". Without
34 the extension the EFI firmware loader will ref 34 the extension the EFI firmware loader will refuse to execute it. It's
35 not possible to execute bzImage.efi from the u 35 not possible to execute bzImage.efi from the usual Linux file systems
36 because EFI firmware doesn't have support for 36 because EFI firmware doesn't have support for them. For ARM the
37 arch/arm/boot/zImage should be copied to the s 37 arch/arm/boot/zImage should be copied to the system partition, and it
38 may not need to be renamed. Similarly for arm6 38 may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
39 should be copied but not necessarily renamed. 39 should be copied but not necessarily renamed.
40 40
41 41
42 Passing kernel parameters from the EFI shell 42 Passing kernel parameters from the EFI shell
43 -------------------------------------------- 43 --------------------------------------------
44 44
45 Arguments to the kernel can be passed after bz 45 Arguments to the kernel can be passed after bzImage.efi, e.g.::
46 46
47 fs0:> bzImage.efi console=ttyS0 root=/ 47 fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
48 48
49 49
50 The "initrd=" option 50 The "initrd=" option
51 -------------------- 51 --------------------
52 52
53 Like most boot loaders, the EFI stub allows th 53 Like most boot loaders, the EFI stub allows the user to specify
54 multiple initrd files using the "initrd=" opti 54 multiple initrd files using the "initrd=" option. This is the only EFI
55 stub-specific command line parameter, everythi 55 stub-specific command line parameter, everything else is passed to the
56 kernel when it boots. 56 kernel when it boots.
57 57
58 The path to the initrd file must be an absolut 58 The path to the initrd file must be an absolute path from the
59 beginning of the ESP, relative path names do n 59 beginning of the ESP, relative path names do not work. Also, the path
60 is an EFI-style path and directory elements mu 60 is an EFI-style path and directory elements must be separated with
61 backslashes (\). For example, given the follow 61 backslashes (\). For example, given the following directory layout::
62 62
63 fs0:> 63 fs0:>
64 Kernels\ 64 Kernels\
65 bzImage.efi 65 bzImage.efi
66 initrd-large.img 66 initrd-large.img
67 67
68 Ramdisks\ 68 Ramdisks\
69 initrd-small.img 69 initrd-small.img
70 initrd-medium.img 70 initrd-medium.img
71 71
72 to boot with the initrd-large.img file if the 72 to boot with the initrd-large.img file if the current working
73 directory is fs0:\Kernels, the following comma 73 directory is fs0:\Kernels, the following command must be used::
74 74
75 fs0:\Kernels> bzImage.efi initrd=\Kern 75 fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
76 76
77 Notice how bzImage.efi can be specified with a 77 Notice how bzImage.efi can be specified with a relative path. That's
78 because the image we're executing is interpret 78 because the image we're executing is interpreted by the EFI shell,
79 which understands relative paths, whereas the 79 which understands relative paths, whereas the rest of the command line
80 is passed to bzImage.efi. 80 is passed to bzImage.efi.
81 81
82 82
83 The "dtb=" option 83 The "dtb=" option
84 ----------------- 84 -----------------
85 85
86 For the ARM and arm64 architectures, a device 86 For the ARM and arm64 architectures, a device tree must be provided to
87 the kernel. Normally firmware shall supply the 87 the kernel. Normally firmware shall supply the device tree via the
88 EFI CONFIGURATION TABLE. However, the "dtb=" c 88 EFI CONFIGURATION TABLE. However, the "dtb=" command line option can
89 be used to override the firmware supplied devi 89 be used to override the firmware supplied device tree, or to supply
90 one when firmware is unable to. 90 one when firmware is unable to.
91 91
92 Please note: Firmware adds runtime configurati 92 Please note: Firmware adds runtime configuration information to the
93 device tree before booting the kernel. If dtb= 93 device tree before booting the kernel. If dtb= is used to override
94 the device tree, then any runtime data provide 94 the device tree, then any runtime data provided by firmware will be
95 lost. The dtb= option should only be used eith 95 lost. The dtb= option should only be used either as a debug tool, or
96 as a last resort when a device tree is not pro 96 as a last resort when a device tree is not provided in the EFI
97 CONFIGURATION TABLE. 97 CONFIGURATION TABLE.
98 98
99 "dtb=" is processed in the same manner as the 99 "dtb=" is processed in the same manner as the "initrd=" option that is
100 described above. 100 described above.
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