Re: [PATCH v2 20/39] xen/riscv: detect Shtvala

[Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>]

On 2026-09-08 12:15 +0200, Oleksii Kurochko wrote:
> 
> 
> On 9/7/26 5:57 PM, Baptiste Le Duc wrote:
> >> Shtvala says that htval is written with the faulting guest physical address
> >> on a guest-page fault. The H extension itself allows an implementation to
> >> write htval with either that address or with zero, so where the extension 
> >> is
> >> absent a zero htval cannot be told apart from a genuine fault on guest
> >> physical address 0-3.
> >>
> >> It is not offered to guests. Shtvala describes the HS-mode trap interface,
> >> which a VS-mode guest never sees, and the H extension it belongs to is
> >> already withheld from guests. Its guest-facing counterpart is a separate
> >> extension, Shvstvala.
> >>
> >> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> >>
> > I thought this commit message is not very clear. Here is a more direct
> > suggestion:
> > 
> > ```
> > The H extension allows htval, on a guest-page fault, to be written with
> > either the faulting guest physical address or zero. Shtvala extension
> > removes the ambiguity of this zero-write by guaranteeing that htval is
> > written with the faulting guest physical address in every circumstance
> > permitted by the ISA.
> > 
> > Not offered to guests: Shtvala describes htval, an HS-mode-only trap
> > register a VS-mode guest never touches and the H extension it belongs to
> > is already hidden from guests. The guest-visible equivalent is a
> > separate extension, Shvstvala, covering vstval instead.
> > ```
> 
> Sounds good to me. I will apply your suggestion.
> 
> > 
> > Btw, I saw Linux has Documentation/devicetree/bindings/riscv/extensions.yaml
> > which describes all of the extensions supported, don't you think it would
> > be useful to have the same in docs/misc/devicetree/...?
> 
> I think it could be useful.
> 
> I think also about booting.txt already exitsted in the codebase. Would 
> you be okay with that? I think I will add the section to booting.txt and 
> pointing to riscv_isa_ext[] in cpufeature.c and so we won't miss an 
> update of doc if we will add or remove support of an extension. Does it 
> sound good to you?
> 
Yes it sounds okay!
Thanks
> ~ Oleksii
> 
> 
> 
>