efirt: add hooks for diverging EFI implementations

[royger]

Summary

Introduce a set of hooks for MI EFI public functions, so that a new
implementation can be done. This will be used to implement the Xen PV
EFI interface that's used when running FreeBSD as a Xen dom0 from UEFI
firmware. Also make the efi_status_to_errno non-static since it will
be used to evaluate status return values from the PV interface.

No functional change indented.

Sponsored by: Citrix Systems R&D

[royger]

For the reference, the user of this new interface is at: https://reviews.freebsd.org/D28621

[imp]

Seems fine. The log could be a little clearer that FreeBSD needs to implement the EFI interfaces for pvefi, but if you can't make it clearer both the code and commit message are OK. And a comment where suggested would give the reader the proper context for the 'why' which wasn't immediately obvious to me.

sys/sys/efi.h

182–201

Might make sense to add that we either call the real efi firmware, or virtualize it for VMs in some kind of comment here.

[kib]

sys/dev/efidev/efirt.c

568

Should this be const.

I prefer to have this structure static, and use const struct efi_ops *active_efi_ops or such for calls, initialized at boot or module load.

[royger]

• Add comment in efi_ops struct to describe the intended usage.
• Introduce active_efi_ops as a pointer to the currently in-use efi_ops.