Re: [RISC-V] Switch to H-mode

[Bobby Eshleman <bobbyeshleman@xxxxxxxxx>]

On Mon, Jan 23, 2023 at 06:56:19PM +0200, Oleksii wrote:
> 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
> 

Ah, what you are seeing there is that the openSBI's Next Mode excludes
the virtualization mode (it treats HS and S synonymously) and it is only
used for setting the mstatus MPP. The code also has next_virt for
setting the MPV but I don't think that is exposed via the device tree
yet. For Xen, you'd want next_mode = PRIV_S and next_virt = 0 (HS mode,
not VS mode). The relevant setup prior to mret is here for interested
readers:
https://github.com/riscv-software-src/opensbi/blob/001106d19b21cd6443ae7f7f6d4d048d80e9ecac/lib/sbi/sbi_hart.c#L759

As long as the next_mode and next_virt are set correctly, then Xen
should be launching in HS mode. I do believe this should be default for
the stock build too for Domain0, unless something has changed.

Thanks,
Bobby