efi-stub.rst

  1=================
  2The EFI Boot Stub
  3=================
  4
  5On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
  6as a PE/COFF image, thereby convincing EFI firmware loaders to load
  7it as an EFI executable. The code that modifies the bzImage header,
  8along with the EFI-specific entry point that the firmware loader
  9jumps to are collectively known as the "EFI boot stub", and live in
 10arch/x86/boot/header.S and drivers/firmware/efi/libstub/x86-stub.c,
 11respectively. For ARM the EFI stub is implemented in
 12arch/arm/boot/compressed/efi-header.S and
 13drivers/firmware/efi/libstub/arm32-stub.c. EFI stub code that is shared
 14between architectures is in drivers/firmware/efi/libstub.
 15
 16For arm64, there is no compressed kernel support, so the Image itself
 17masquerades as a PE/COFF image and the EFI stub is linked into the
 18kernel. The arm64 EFI stub lives in drivers/firmware/efi/libstub/arm64.c
 19and drivers/firmware/efi/libstub/arm64-stub.c.
 20
 21By using the EFI boot stub it's possible to boot a Linux kernel
 22without the use of a conventional EFI boot loader, such as grub or
 23elilo. Since the EFI boot stub performs the jobs of a boot loader, in
 24a certain sense it *IS* the boot loader.
 25
 26The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
 27
 28
 29How to install bzImage.efi
 30--------------------------
 31
 32The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
 33System Partition (ESP) and renamed with the extension ".efi". Without
 34the extension the EFI firmware loader will refuse to execute it. It's
 35not possible to execute bzImage.efi from the usual Linux file systems
 36because EFI firmware doesn't have support for them. For ARM the
 37arch/arm/boot/zImage should be copied to the system partition, and it
 38may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
 39should be copied but not necessarily renamed.
 40
 41
 42Passing kernel parameters from the EFI shell
 43--------------------------------------------
 44
 45Arguments to the kernel can be passed after bzImage.efi, e.g.::
 46
 47	fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
 48
 49
 50The "initrd=" option
 51--------------------
 52
 53Like most boot loaders, the EFI stub allows the user to specify
 54multiple initrd files using the "initrd=" option. This is the only EFI
 55stub-specific command line parameter, everything else is passed to the
 56kernel when it boots.
 57
 58The path to the initrd file must be an absolute path from the
 59beginning of the ESP, relative path names do not work. Also, the path
 60is an EFI-style path and directory elements must be separated with
 61backslashes (\). For example, given the following directory layout::
 62
 63  fs0:>
 64	Kernels\
 65			bzImage.efi
 66			initrd-large.img
 67
 68	Ramdisks\
 69			initrd-small.img
 70			initrd-medium.img
 71
 72to boot with the initrd-large.img file if the current working
 73directory is fs0:\Kernels, the following command must be used::
 74
 75	fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
 76
 77Notice how bzImage.efi can be specified with a relative path. That's
 78because the image we're executing is interpreted by the EFI shell,
 79which understands relative paths, whereas the rest of the command line
 80is passed to bzImage.efi.
 81
 82.. hint::
 83   It is also possible to provide an initrd using a Linux-specific UEFI
 84   protocol at boot time. See :ref:`pe-coff-entry-point` for details.
 85
 86The "dtb=" option
 87-----------------
 88
 89For the ARM and arm64 architectures, a device tree must be provided to
 90the kernel. Normally firmware shall supply the device tree via the
 91EFI CONFIGURATION TABLE. However, the "dtb=" command line option can
 92be used to override the firmware supplied device tree, or to supply
 93one when firmware is unable to.
 94
 95Please note: Firmware adds runtime configuration information to the
 96device tree before booting the kernel. If dtb= is used to override
 97the device tree, then any runtime data provided by firmware will be
 98lost. The dtb= option should only be used either as a debug tool, or
 99as a last resort when a device tree is not provided in the EFI
100CONFIGURATION TABLE.
101
102"dtb=" is processed in the same manner as the "initrd=" option that is
103described above.