| The EFI Boot Stub |
| --------------------------- |
| |
| On the x86 platform, a bzImage can masquerade as a PE/COFF image, |
| thereby convincing EFI firmware loaders to load it as an EFI |
| executable. The code that modifies the bzImage header, along with the |
| EFI-specific entry point that the firmware loader jumps to are |
| collectively known as the "EFI boot stub", and live in |
| arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c, |
| respectively. |
| |
| By using the EFI boot stub it's possible to boot a Linux kernel |
| without the use of a conventional EFI boot loader, such as grub or |
| elilo. Since the EFI boot stub performs the jobs of a boot loader, in |
| a certain sense it *IS* the boot loader. |
| |
| The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option. |
| |
| |
| **** How to install bzImage.efi |
| |
| The bzImage located in arch/x86/boot/bzImage must be copied to the EFI |
| System Partition (ESP) and renamed with the extension ".efi". Without |
| the extension the EFI firmware loader will refuse to execute it. It's |
| not possible to execute bzImage.efi from the usual Linux file systems |
| because EFI firmware doesn't have support for them. |
| |
| |
| **** Passing kernel parameters from the EFI shell |
| |
| Arguments to the kernel can be passed after bzImage.efi, e.g. |
| |
| fs0:> bzImage.efi console=ttyS0 root=/dev/sda4 |
| |
| |
| **** The "initrd=" option |
| |
| Like most boot loaders, the EFI stub allows the user to specify |
| multiple initrd files using the "initrd=" option. This is the only EFI |
| stub-specific command line parameter, everything else is passed to the |
| kernel when it boots. |
| |
| The path to the initrd file must be an absolute path from the |
| beginning of the ESP, relative path names do not work. Also, the path |
| is an EFI-style path and directory elements must be separated with |
| backslashes (\). For example, given the following directory layout, |
| |
| fs0:> |
| Kernels\ |
| bzImage.efi |
| initrd-large.img |
| |
| Ramdisks\ |
| initrd-small.img |
| initrd-medium.img |
| |
| to boot with the initrd-large.img file if the current working |
| directory is fs0:\Kernels, the following command must be used, |
| |
| fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img |
| |
| Notice how bzImage.efi can be specified with a relative path. That's |
| because the image we're executing is interpreted by the EFI shell, |
| which understands relative paths, whereas the rest of the command line |
| is passed to bzImage.efi. |
