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diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/admin-guide/efi-stub.rst new file mode 100644 index 000000000..b24e7c40d --- /dev/null +++ b/Documentation/admin-guide/efi-stub.rst @@ -0,0 +1,100 @@ +================= +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 drivers/firmware/efi/libstub/x86-stub.c, +respectively. For ARM the EFI stub is implemented in +arch/arm/boot/compressed/efi-header.S and +drivers/firmware/efi/libstub/arm32-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, a device tree must be provided to +the kernel. Normally firmware shall supply the device tree via the +EFI CONFIGURATION TABLE. However, the "dtb=" command line option can +be used to override the firmware supplied device tree, or to supply +one when firmware is unable to. + +Please note: Firmware adds runtime configuration information to the +device tree before booting the kernel. If dtb= is used to override +the device tree, then any runtime data provided by firmware will be +lost. The dtb= option should only be used either as a debug tool, or +as a last resort when a device tree is not provided in the EFI +CONFIGURATION TABLE. + +"dtb=" is processed in the same manner as the "initrd=" option that is +described above. |