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 drivers/firmware/efi/libstub/x86-stub.c, 11 respectively. For ARM the EFI stub is implemented in 12 arch/arm/boot/compressed/efi-header.S and 13 drivers/firmware/efi/libstub/arm32-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 drivers/firmware/efi/libstub/arm64.c 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, a device tree must be provided to 87 the kernel. Normally firmware shall supply the device tree via the 88 EFI CONFIGURATION TABLE. However, the "dtb=" command line option can 89 be used to override the firmware supplied device tree, or to supply 90 one when firmware is unable to. 91 92 Please note: Firmware adds runtime configuration information to the 93 device tree before booting the kernel. If dtb= is used to override 94 the device tree, then any runtime data provided by firmware will be 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 provided in the EFI 97 CONFIGURATION TABLE. 98 99 "dtb=" is processed in the same manner as the "initrd=" option that is 100 described above.
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