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| 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. |
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