UEFI support in ARM DomUs

[Roman Shaposhnik <roman@zededa.com>]

Hi!

with a lot of help from Stefano, we're getting RPi4 support in
Project EVE pretty much on par between KVM and Xen.

One big area that still remains is supporting UEFI boot sequence
for DomUs. With KVM, given the qemu virt device model this is
as simple as using either stock UEFI build for arm or even U-Boot
EFI emulation environment and passing it via -bios option.

Obviously with Xen on ARM we don't have the device model so
my understanding is that the easiest way we can support it would
be to port UEFI's OvmfPkg/OvmfXen target to ARM (it seems to
be currently exclusively X64).

So here's my first question: if there's anybody on this list who had
a hand in implementing OvmfPkg/OvmfXen can you please share
your thoughts on how much work that port may be (or whether it is
even feasible -- I may totally be missing something really obvious here).

And as long as I've got your attention: two more questions:
   1.. compared to the above, would porting pvgrub to ARM be any
   easier or more difficult?

   2. same question for teaching u-boot about PV calls.

Thanks,
Roman.

P.S. Oh and I guess between:
   0. OvmfPkg/OvmfXen on ARM64
   1. pvgrub on ARM64
   2. u-boot/EFI emulation with PV calls backend
I didn't miss any other obvious way of making UEFI-aware VM images
to boot on Xen ARM64 DomU, right?