[RISC-V] Switch to H-mode

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

Hi Alistair and community,

I am working on RISC-V support upstream for Xen based on your and Bobby
patches.

Adding the RISC-V support I realized that Xen is ran in S-mode. Output
of OpenSBI:
    ...
    Domain0 Next Mode         : S-mode
    ...
So the first my question is shouldn't it be in H-mode?

If I am right than it looks like we have to do a patch to OpenSBI to
add support of H-mode as it is not supported now:
[1]
https://github.com/riscv-software-src/opensbi/blob/master/lib/sbi/sbi_domain.c#L380
[2]
https://github.com/riscv-software-src/opensbi/blob/master/include/sbi/riscv_encoding.h#L110
Please correct me if I am wrong.

The other option I see is to switch to H-mode in U-boot as I understand
the classical boot flow is:
    OpenSBI -> U-boot -> Xen -> Domain{0,...}
If it is at all possible since U-boot will be in S mode after OpenSBI.

Thanks in advance.

~ Oleksii