Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.
1 The EFI Boot Stub 2 --------------------------- 3 4 On the x86 platform, a bzImage can masquerade as a PE/COFF image, 5 thereby convincing EFI firmware loaders to load it as an EFI 6 executable. The code that modifies the bzImage header, along with the 7 EFI-specific entry point that the firmware loader jumps to are 8 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. 11 12 By using the EFI boot stub it's possible to boot a Linux kernel 13 without the use of a conventional EFI boot loader, such as grub or 14 elilo. Since the EFI boot stub performs the jobs of a boot loader, in 15 a certain sense it *IS* the boot loader. 16 17 The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. 18 19 20 **** How to install bzImage.efi 21 22 The bzImage located in arch/x86/boot/bzImage must be copied to the EFI 23 System Partiion (ESP) and renamed with the extension ".efi". Without 24 the extension the EFI firmware loader will refuse to execute it. It's 25 not possible to execute bzImage.efi from the usual Linux file systems 26 because EFI firmware doesn't have support for them. 27 28 29 **** Passing kernel parameters from the EFI shell 30 31 Arguments 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 38 Like most boot loaders, the EFI stub allows the user to specify 39 multiple initrd files using the "initrd=" option. This is the only EFI 40 stub-specific command line parameter, everything else is passed to the 41 kernel when it boots. 42 43 The path to the initrd file must be an absolute path from the 44 beginning of the ESP, relative path names do not work. Also, the path 45 is an EFI-style path and directory elements must be separated with 46 backslashes (\). For example, given the following directory layout, 47 48 fs0:> 49 Kernels\ 50 bzImage.efi 51 initrd-large.img 52 53 Ramdisks\ 54 initrd-small.img 55 initrd-medium.img 56 57 to boot with the initrd-large.img file if the current working 58 directory is fs0:\Kernels, the following command must be used, 59 60 fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img 61 62 Notice how bzImage.efi can be specified with a relative path. That's 63 because the image we're executing is interpreted by the EFI shell, 64 which understands relative paths, whereas the rest of the command line 65 is passed to bzImage.efi.