path: root/Documentation/efi-stub.txt
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authorRoy Franz <roy.franz@linaro.org>2013-09-22 15:45:25 -0700
committerMatt Fleming <matt.fleming@intel.com>2013-09-25 12:34:32 +0100
commit4172fe2f8a479e2237459918edc83b027efa8808 (patch)
tree8db815fdd21ffc767ac77e3701f54eb1465be9c6 /Documentation/efi-stub.txt
parent258f6fd738221766b512cd8c7120563b78d62829 (diff)
EFI stub documentation updates
Move efi-stub.txt out of x86 directory and into common directory in preparation for adding ARM EFI stub support. Signed-off-by: Roy Franz <roy.franz@linaro.org> Signed-off-by: Matt Fleming <matt.fleming@intel.com>
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+ The EFI Boot Stub
+ ---------------------------
+On the x86 platform, a 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,
+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 Partiion (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.
+**** 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,
+ 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.

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