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0001                           The EFI Boot Stub
0002                      ---------------------------
0003 
0004 On the x86 and ARM platforms, a kernel zImage/bzImage can masquerade
0005 as a PE/COFF image, thereby convincing EFI firmware loaders to load
0006 it as an EFI executable. The code that modifies the bzImage header,
0007 along with the EFI-specific entry point that the firmware loader
0008 jumps to are collectively known as the "EFI boot stub", and live in
0009 arch/x86/boot/header.S and arch/x86/boot/compressed/eboot.c,
0010 respectively. For ARM the EFI stub is implemented in
0011 arch/arm/boot/compressed/efi-header.S and
0012 arch/arm/boot/compressed/efi-stub.c. EFI stub code that is shared
0013 between architectures is in drivers/firmware/efi/libstub.
0014 
0015 For arm64, there is no compressed kernel support, so the Image itself
0016 masquerades as a PE/COFF image and the EFI stub is linked into the
0017 kernel. The arm64 EFI stub lives in arch/arm64/kernel/efi-entry.S
0018 and drivers/firmware/efi/libstub/arm64-stub.c.
0019 
0020 By using the EFI boot stub it's possible to boot a Linux kernel
0021 without the use of a conventional EFI boot loader, such as grub or
0022 elilo. Since the EFI boot stub performs the jobs of a boot loader, in
0023 a certain sense it *IS* the boot loader.
0024 
0025 The EFI boot stub is enabled with the CONFIG_EFI_STUB kernel option.
0026 
0027 
0028 **** How to install bzImage.efi
0029 
0030 The bzImage located in arch/x86/boot/bzImage must be copied to the EFI
0031 System Partition (ESP) and renamed with the extension ".efi". Without
0032 the extension the EFI firmware loader will refuse to execute it. It's
0033 not possible to execute bzImage.efi from the usual Linux file systems
0034 because EFI firmware doesn't have support for them. For ARM the
0035 arch/arm/boot/zImage should be copied to the system partition, and it
0036 may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
0037 should be copied but not necessarily renamed.
0038 
0039 
0040 **** Passing kernel parameters from the EFI shell
0041 
0042 Arguments to the kernel can be passed after bzImage.efi, e.g.
0043 
0044         fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
0045 
0046 
0047 **** The "initrd=" option
0048 
0049 Like most boot loaders, the EFI stub allows the user to specify
0050 multiple initrd files using the "initrd=" option. This is the only EFI
0051 stub-specific command line parameter, everything else is passed to the
0052 kernel when it boots.
0053 
0054 The path to the initrd file must be an absolute path from the
0055 beginning of the ESP, relative path names do not work. Also, the path
0056 is an EFI-style path and directory elements must be separated with
0057 backslashes (\). For example, given the following directory layout,
0058 
0059 fs0:>
0060         Kernels\
0061                         bzImage.efi
0062                         initrd-large.img
0063 
0064         Ramdisks\
0065                         initrd-small.img
0066                         initrd-medium.img
0067 
0068 to boot with the initrd-large.img file if the current working
0069 directory is fs0:\Kernels, the following command must be used,
0070 
0071         fs0:\Kernels> bzImage.efi initrd=\Kernels\initrd-large.img
0072 
0073 Notice how bzImage.efi can be specified with a relative path. That's
0074 because the image we're executing is interpreted by the EFI shell,
0075 which understands relative paths, whereas the rest of the command line
0076 is passed to bzImage.efi.
0077 
0078 
0079 **** The "dtb=" option
0080 
0081 For the ARM and arm64 architectures, we also need to be able to provide a
0082 device tree to the kernel. This is done with the "dtb=" command line option,
0083 and is processed in the same manner as the "initrd=" option that is
0084 described above.