Documentation/admin-guide/efi-stub.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/efi-stub.rst

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Linux kernel
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Documentation/admin-guide/efi-stub.rst
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Support Tooling And Documentation
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Documentation
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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=================
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 drivers/firmware/efi/libstub/arm64.c
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

Annotation

Implementation Notes