1 The EFI Boot Stub 2 --------------------------- 3 4On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade 5as a PE/COFF image, thereby convincing EFI firmware loaders to load 6it as an EFI executable. The code that modifies the bzImage header, 7along with the EFI-specific entry point that the firmware loader 8jumps to are collectively known as the "EFI boot stub", and live in 9arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c, 10respectively. For ARM the EFI stub is implemented in 11arch/arm/boot/compressed/efi-header.S and 12arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared 13between architectures is in drivers/firmware/efi/efi-stub-helper.c. 14 15For arm64, there is no compressed kernel support, so the Image itself 16masquerades as a PE/COFF image and the EFI stub is linked into the 17kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S 18and arch/arm64/kernel/efi-stub.c. 19 20By using the EFI boot stub it's possible to boot a Linux kernel 21without the use of a conventional EFI boot loader, such as grub or 22elilo. Since the EFI boot stub performs the jobs of a boot loader, in 23a certain sense it *IS* the boot loader. 24 25The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. 26 27 28**** How to install bzImage.efi 29 30The bzImage located in arch/x86/boot/bzImage must be copied to the EFI 31System Partition (ESP) and renamed with the extension ".efi". Without 32the extension the EFI firmware loader will refuse to execute it. It's 33not possible to execute bzImage.efi from the usual Linux file systems 34because EFI firmware doesn't have support for them. For ARM the 35arch/arm/boot/zImage should be copied to the system partition, and it 36may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image 37should be copied but not necessarily renamed. 38 39 40**** Passing kernel parameters from the EFI shell 41 42Arguments 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 49Like most boot loaders, the EFI stub allows the user to specify 50multiple initrd files using the "initrd=" option. This is the only EFI 51stub-specific command line parameter, everything else is passed to the 52kernel when it boots. 53 54The path to the initrd file must be an absolute path from the 55beginning of the ESP, relative path names do not work. Also, the path 56is an EFI-style path and directory elements must be separated with 57backslashes (\). For example, given the following directory layout, 58 59fs0:> 60 Kernels\ 61 bzImage.efi 62 initrd-large.img 63 64 Ramdisks\ 65 initrd-small.img 66 initrd-medium.img 67 68to boot with the initrd-large.img file if the current working 69directory is fs0:\Kernels, the following command must be used, 70 71 fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img 72 73Notice how bzImage.efi can be specified with a relative path. That's 74because the image we're executing is interpreted by the EFI shell, 75which understands relative paths, whereas the rest of the command line 76is passed to bzImage.efi. 77 78 79**** The "dtb=" option 80 81For the ARM and arm64 architectures, we also need to be able to provide a 82device tree to the kernel. This is done with the "dtb=" command line option, 83and is processed in the same manner as the "initrd=" option that is 84described above. 85