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 arch/x86/boot/compressed/eboot.c, 11respectively. For ARM the EFI stub is implemented in 12arch/arm/boot/compressed/efi-header.S and 13arch/arm/boot/compressed/efi-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