0c7596621e
Since we can't expect every user to read the EFI boot stub code it seems prudent to have a couple of paragraphs explaining what it is and how it works. The "initrd=" option in particular is tricky because it only understands absolute EFI-style paths (backslashes as directory separators), and until now this hasn't been documented anywhere. This has tripped up a couple of users. Cc: Matthew Garrett <mjg@redhat.com> Cc: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Matt Fleming <matt.fleming@intel.com> Link: http://lkml.kernel.org/r/1331907517-3985-4-git-send-email-matt@console-pimps.org Signed-off-by: H. Peter Anvin <hpa@zytor.com>
66 lines
2.3 KiB
Plaintext
66 lines
2.3 KiB
Plaintext
The EFI Boot Stub
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On the x86 platform, a bzImage can masquerade as a PE/COFF image,
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thereby convincing EFI firmware loaders to load it as an EFI
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executable. The code that modifies the bzImage header, along with the
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EFI-specific entry point that the firmware loader jumps to are
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collectively known as the "EFI boot stub", and live in
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arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
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respectively.
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By using the EFI boot stub it's possible to boot a Linux kernel
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without the use of a conventional EFI boot loader, such as grub or
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elilo. Since the EFI boot stub performs the jobs of a boot loader, in
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a certain sense it *IS* the boot loader.
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The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
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**** How to install bzImage.efi
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The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
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System Partiion (ESP) and renamed with the extension ".efi". Without
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the extension the EFI firmware loader will refuse to execute it. It's
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not possible to execute bzImage.efi from the usual Linux file systems
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because EFI firmware doesn't have support for them.
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**** Passing kernel parameters from the EFI shell
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Arguments to the kernel can be passed after bzImage.efi, e.g.
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fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
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**** The "initrd=" option
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Like most boot loaders, the EFI stub allows the user to specify
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multiple initrd files using the "initrd=" option. This is the only EFI
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stub-specific command line parameter, everything else is passed to the
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kernel when it boots.
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The path to the initrd file must be an absolute path from the
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beginning of the ESP, relative path names do not work. Also, the path
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is an EFI-style path and directory elements must be separated with
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backslashes (\). For example, given the following directory layout,
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fs0:>
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Kernels\
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bzImage.efi
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initrd-large.img
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Ramdisks\
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initrd-small.img
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initrd-medium.img
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to boot with the initrd-large.img file if the current working
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directory is fs0:\Kernels, the following command must be used,
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fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
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Notice how bzImage.efi can be specified with a relative path. That's
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because the image we're executing is interpreted by the EFI shell,
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which understands relative paths, whereas the rest of the command line
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is passed to bzImage.efi.
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