arch-agnostic initrd loading method for EFI systems

This series introduces an arch agnostic way of loading the initrd into memory from the EFI stub. This addresses a number of shortcomings that affect the current implementations that exist across architectures:

- The initrd=<file> command line option can only load files that reside on the same file system that the kernel itself was loaded from, which requires the bootloader or firmware to expose that file system via the appropriate EFI protocol, which is not always feasible. From the kernel side, this protocol is problematic since it is incompatible with mixed mode on x86 (this is due to the fact that some of its methods have prototypes that are difficult to marshall)

- The approach that is ordinarily taken by GRUB is to load the initrd into memory, and pass it to the kernel proper via the bootparams structure or via the device tree. This requires the boot loader to have an understanding of those structures, which are not always set in stone, and of the policies around where the initrd may be loaded into memory. In the ARM case, it requires GRUB to modify the hardware description provided by the firmware, given that the initrd base and offset in memory are passed via the same data structure. It also creates a time window where the initrd data sits in memory, and can potentially be corrupted before the kernel is booted.

Considering that we will soon have new users of these interfaces (EFI for kvmtool on ARM, RISC-V in u-boot, etc), it makes sense to add a generic interface now, before having another wave of bespoke arch specific code coming in.

Another aspect to take into account is that support for UEFI secure boot and measured boot is being taken into the upstream, and being able to rely on the PE entry point for booting any architecture makes the GRUB vs shim story much cleaner, as we should be able to rely on LoadImage and StartImage on all architectures, while retaining the ability to load initrds from anywhere.

Note that these patches depend on a fair amount of cleanup work that I am targetting for v5.7. Branch can be found at:
https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git/log/?h=next

A PoC implementation for OVMF and ArmVirtQemu (OVMF for ARM aka AAVMF) can be found at https://github.com/ardbiesheuvel/edk2/commits/linux-efi-generic.

A U-Boot implementation is under development as well, and can be found at
https://github.com/apalos/u-boot/commits/efi_load_file_8

Changes since v1:
- merge vendor media device path type definition with the existing device path definitions we already have, and rework the latter slightly to be more easily reusable
- use 'dev_path' not 'devpath' consistently
- pass correct FilePath value to LoadFile2 (i.e., the device path pointer that was advanced to point to the 'end' node by locate_device_path())
- add kerneldoc comment to efi_load_initrd_dev_path()
- take care to only return EFI_NOT_FOUND from efi_load_initrd_dev_path() if the LoadFile2 protocol does not exist on the LINUX_EFI_INITRD_MEDIA_GUID device path - this makes the logic whether to fallback to the command line method more robust

Ard Biesheuvel (3):
efi/dev-path-parser: Add struct definition for vendor type device path
nodes
efi/libstub: Add support for loading the initrd from a device path efi/libstub: Take noinitrd cmdline argument into account for devpath
initrd

 drivers/firmware/efi/apple-properties.c       |  8 +-
 drivers/firmware/efi/dev-path-parser.c        | 38 ++++----
 drivers/firmware/efi/libstub/arm-stub.c       | 20 ++++-
 .../firmware/efi/libstub/efi-stub-helper.c    | 89 +++++++++++++++++++
 drivers/firmware/efi/libstub/efistub.h        |  5 ++
 drivers/firmware/efi/libstub/x86-stub.c       | 47 ++++++++--
 include/linux/efi.h                           | 49 ++++++----
 7 files changed, 201 insertions(+), 55 deletions(-)

2.17.1