101 lines
3.8 KiB
ReStructuredText
101 lines
3.8 KiB
ReStructuredText
|
=================
|
||
|
|
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, 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.
|