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