efi: Bump stub image version for macOS HVF compatibility

efi: Bump stub image version for macOS HVF compatibility

The macOS hypervisor framework includes a host-side VMM called
VZLinuxBootLoader [1] which implements native support for booting the
Linux kernel inside a guest directly (instead of, e.g., via GRUB
installed inside the guest). On x86, it incorporates a BIOS style loader
that does not implement or expose EFI to the loaded kernel. However,
this loader appears to fail when the 'image minor version' field in the
kernel image's PE/COFF header (which is generally only used by EFI based
bootloaders) is set to any value other than 0x0. [2]

Commit e346bebbd36b1576 ("efi: libstub: Always enable initrd command
line loader and bump version") incremented the EFI stub image minor
version to convey that all EFI stub kernels now implement support for
the initrd= command line option, and do so in a way where it can load
initrd images from any filesystem known to the EFI firmware (as opposed
to prior implementations that could only load initrds from the same
volume that the kernel image was loaded from).

Unfortunately, bumping the version to v1.1 triggers this issue in
VZLinuxBootLoader, breaking the boot on x86. So let's keep the image
minor version at 0x0, and bump the image major version instead.

While at it, convert this field to a bit field, so that individual
features are discoverable from it, as suggested by Linus. So let's bump
the major version to v3, and document the initrd= command line loading
feature as being represented by bit 1 in the mask.

Note that, due to the prior interpretation as a monotonically increasing
version field, loaders are still permitted to assume that the LoadFile2
initrd loading feature is supported for any major version value >= 1,
even if bit 0 is not set.

[1] https://developer.apple.com/documentation/virtualization/vzlinuxbootloader
[2] https://lore.kernel.org/linux-efi/CAG8fp8Teu4G9JuenQrqGndFt2Gy+V4YgJ=hN1xX7AD940YKf3A@mail.gmail.com/

Fixes: e346bebbd36b1576 ("efi: libstub: Always enable initrd command ...")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217485
Signed-off-by: Akihiro Suda <suda.kyoto@gmail.com>
[ardb: rewrite comment and commit log]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>