1 The EFI Boot Stub 2 --------------------------- 3 4On the x86 platform, a bzImage can masquerade as a PE/COFF image, 5thereby convincing EFI firmware loaders to load it as an EFI 6executable. The code that modifies the bzImage header, along with the 7EFI-specific entry point that the firmware loader jumps to are 8collectively known as the "EFI boot stub", and live in 9arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c, 10respectively. 11 12By using the EFI boot stub it's possible to boot a Linux kernel 13without the use of a conventional EFI boot loader, such as grub or 14elilo. Since the EFI boot stub performs the jobs of a boot loader, in 15a certain sense it *IS* the boot loader. 16 17The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. 18 19 20**** How to install bzImage.efi 21 22The bzImage located in arch/x86/boot/bzImage must be copied to the EFI 23System Partiion (ESP) and renamed with the extension ".efi". Without 24the extension the EFI firmware loader will refuse to execute it. It's 25not possible to execute bzImage.efi from the usual Linux file systems 26because EFI firmware doesn't have support for them. 27 28 29**** Passing kernel parameters from the EFI shell 30 31Arguments to the kernel can be passed after bzImage.efi, e.g. 32 33 fs0:> bzImage.efi console=ttyS0 root=/dev/sda4 34 35 36**** The "initrd=" option 37 38Like most boot loaders, the EFI stub allows the user to specify 39multiple initrd files using the "initrd=" option. This is the only EFI 40stub-specific command line parameter, everything else is passed to the 41kernel when it boots. 42 43The path to the initrd file must be an absolute path from the 44beginning of the ESP, relative path names do not work. Also, the path 45is an EFI-style path and directory elements must be separated with 46backslashes (\). For example, given the following directory layout, 47 48fs0:> 49 Kernels\ 50 bzImage.efi 51 initrd-large.img 52 53 Ramdisks\ 54 initrd-small.img 55 initrd-medium.img 56 57to boot with the initrd-large.img file if the current working 58directory is fs0:\Kernels, the following command must be used, 59 60 fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img 61 62Notice how bzImage.efi can be specified with a relative path. That's 63because the image we're executing is interpreted by the EFI shell, 64which understands relative paths, whereas the rest of the command line 65is passed to bzImage.efi. 66