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

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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@gmail.com>
>>
> 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?

~ Oleksii