| The EFI Boot Stub | |
| --------------------------- | |
| On the x86 and ARM platforms, a kernel zImage/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. For ARM the EFI stub is implemented in | |
| arch/arm/boot/compressed/efi-header.S and | |
| arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared | |
| between architectures is in drivers/firmware/efi/libstub. | |
| For arm64, there is no compressed kernel support, so the Image itself | |
| masquerades as a PE/COFF image and the EFI stub is linked into the | |
| kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S | |
| and drivers/firmware/efi/libstub/arm64-stub.c. | |
| 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. For ARM the | |
| arch/arm/boot/zImage should be copied to the system partition, and it | |
| may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image | |
| should be copied but not necessarily renamed. | |
| **** 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. | |
| **** The "dtb=" option | |
| For the ARM and arm64 architectures, we also need to be able to provide a | |
| device tree to the kernel. This is done with the "dtb=" command line option, | |
| and is processed in the same manner as the "initrd=" option that is | |
| described above. |