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

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Thu, 2023-01-19 at 13:05 +0000, Bobby Eshleman wrote:
> 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.
> 
The same I've found in OpenSBI before but wasn't 100% sure that I'm
right. Now it is clear.

Thanks for your explanation.

> Thanks,
> Bobby

~ Oleksii