1=================
2The EFI Boot Stub
3=================
4
5On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
6as a PE/COFF image, thereby convincing EFI firmware loaders to load
7it as an EFI executable. The code that modifies the bzImage header,
8along with the EFI-specific entry point that the firmware loader
9jumps to are collectively known as the "EFI boot stub", and live in
10arch/x86/boot/header.S and drivers/firmware/efi/libstub/x86-stub.c,
11respectively. For ARM the EFI stub is implemented in
12arch/arm/boot/compressed/efi-header.S and
13drivers/firmware/efi/libstub/arm32-stub.c. EFI stub code that is shared
14between architectures is in drivers/firmware/efi/libstub.
15
16For arm64, there is no compressed kernel support, so the Image itself
17masquerades as a PE/COFF image and the EFI stub is linked into the
18kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
19and drivers/firmware/efi/libstub/arm64-stub.c.
20
21By using the EFI boot stub it's possible to boot a Linux kernel
22without the use of a conventional EFI boot loader, such as grub or
23elilo. Since the EFI boot stub performs the jobs of a boot loader, in
24a certain sense it *IS* the boot loader.
25
26The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
27
28
29How to install bzImage.efi
30--------------------------
31
32The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
33System Partition (ESP) and renamed with the extension ".efi". Without
34the extension the EFI firmware loader will refuse to execute it. It's
35not possible to execute bzImage.efi from the usual Linux file systems
36because EFI firmware doesn't have support for them. For ARM the
37arch/arm/boot/zImage should be copied to the system partition, and it
38may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
39should be copied but not necessarily renamed.
40
41
42Passing kernel parameters from the EFI shell
43--------------------------------------------
44
45Arguments to the kernel can be passed after bzImage.efi, e.g.::
46
47	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
48
49
50The "initrd=" option
51--------------------
52
53Like most boot loaders, the EFI stub allows the user to specify
54multiple initrd files using the "initrd=" option. This is the only EFI
55stub-specific command line parameter, everything else is passed to the
56kernel when it boots.
57
58The path to the initrd file must be an absolute path from the
59beginning of the ESP, relative path names do not work. Also, the path
60is an EFI-style path and directory elements must be separated with
61backslashes (\). 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
72to boot with the initrd-large.img file if the current working
73directory is fs0:\Kernels, the following command must be used::
74
75	fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
76
77Notice how bzImage.efi can be specified with a relative path. That's
78because the image we're executing is interpreted by the EFI shell,
79which understands relative paths, whereas the rest of the command line
80is passed to bzImage.efi.
81
82
83The "dtb=" option
84-----------------
85
86For the ARM and arm64 architectures, a device tree must be provided to
87the kernel. Normally firmware shall supply the device tree via the
88EFI CONFIGURATION TABLE. However, the "dtb=" command line option can
89be used to override the firmware supplied device tree, or to supply
90one when firmware is unable to.
91
92Please note: Firmware adds runtime configuration information to the
93device tree before booting the kernel. If dtb= is used to override
94the device tree, then any runtime data provided by firmware will be
95lost. The dtb= option should only be used either as a debug tool, or
96as a last resort when a device tree is not provided in the EFI
97CONFIGURATION TABLE.
98
99"dtb=" is processed in the same manner as the "initrd=" option that is
100described above.
101