mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-11-01 17:08:10 +00:00
cdd7857898
Add explanation of arm64 EFI stub and kernel image header changes needed to masquerade as a PE/COFF application. Signed-off-by: Mark Salter <msalter@redhat.com> Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org> Acked-by: Grant Likely <grant.likely@linaro.org> Acked-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Matt Fleming <matt.fleming@intel.com>
84 lines
3.2 KiB
Text
84 lines
3.2 KiB
Text
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/efi-stub-helper.c.
|
|
|
|
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 arch/arm64/kernel/efi-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.
|