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

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

> 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.
```

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/...?

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