Re: [PATCH v5 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

[Alexandre Ghiti]

On 20/06/2024 08:25, Anup Patel wrote:
> On Wed, Jun 5, 2024 at 10:25 PM Conor Dooley <conor@xxxxxxxxxx> wrote:
> > On Wed, Jun 05, 2024 at 08:15:08PM +0800, Yong-Xuan Wang wrote:
> > > Add entries for the Svade and Svadu extensions to the riscv,isa-extensions
> > > property.
> > >
> > > Signed-off-by: Yong-Xuan Wang <yongxuan.wang@xxxxxxxxxx>
> > > ---
> > >   .../devicetree/bindings/riscv/extensions.yaml | 30 +++++++++++++++++++
> > >   1 file changed, 30 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
> > > index 468c646247aa..1e30988826b9 100644
> > > --- a/Documentation/devicetree/bindings/riscv/extensions.yaml
> > > +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
> > > @@ -153,6 +153,36 @@ properties:
> > >               ratified at commit 3f9ed34 ("Add ability to manually trigger
> > >               workflow. (#2)") of riscv-time-compare.
> > >
> > > +        - const: svade
> > > +          description: |
> > > +            The standard Svade supervisor-level extension for raising page-fault
> > > +            exceptions when PTE A/D bits need be set as ratified in the 20240213
> > > +            version of the privileged ISA specification.
> > > +
> > > +            Both Svade and Svadu extensions control the hardware behavior when
> > > +            the PTE A/D bits need to be set. The default behavior for the four
> > > +            possible combinations of these extensions in the device tree are:
> > > +            1. Neither svade nor svadu in DT: default to svade.
> > I think this needs to be expanded on, as to why nothing means svade.
> Actually if both Svade and Svadu are not present in DT then
> it is left to the platform and OpenSBI does nothing.
>
> > > +            2. Only svade in DT: use svade.
> > That's a statement of the obvious, right?
> >
> > > +            3. Only svadu in DT: use svadu.
> > This is not relevant for Svade.
> >
> > > +            4. Both svade and svadu in DT: default to svade (Linux can switch to
> > > +               svadu once the SBI FWFT extension is available).
> > "The privilege level to which this devicetree has been provided can switch to
> > Svadu if the SBI FWFT extension is available".
> >
> > > +        - const: svadu
> > > +          description: |
> > > +            The standard Svadu supervisor-level extension for hardware updating
> > > +            of PTE A/D bits as ratified at commit c1abccf ("Merge pull request
> > > +            #25 from ved-rivos/ratified") of riscv-svadu.
> > > +
> > > +            Both Svade and Svadu extensions control the hardware behavior when
> > > +            the PTE A/D bits need to be set. The default behavior for the four
> > > +            possible combinations of these extensions in the device tree are:
> > @Anup/Drew/Alex, are we missing some wording in here about it only being
> > valid to have Svadu in isolation if the provider of the devicetree has
> > actually turned on Svadu? The binding says "the default behaviour", but
> > it is not the "default" behaviour, the behaviour is a must AFAICT. If
> > you set Svadu in isolation, you /must/ have turned it on. If you set
> > Svadu and Svade, you must have Svadu turned off?
> Yes, the wording should be more of requirement style using
> must or may.
>
> How about this ?
> 1) Both Svade and Svadu not present in DT => Supervisor may
>      assume Svade to be present and enabled or it can discover
>      based on mvendorid, marchid, and mimpid.
> 2) Only Svade present in DT => Supervisor must assume Svade
>      to be always enabled. (Obvious)
> 3) Only Svadu present in DT => Supervisor must assume Svadu
>      to be always enabled. (Obvious)


I agree with all of that, but the problem is how can we guarantee that 
openSBI actually enabled svadu? This is not the case for now.


> 4) Both Svade and Svadu present in DT => Supervisor must
>      assume Svadu turned-off at boot time. To use Svadu, supervisor
>      must explicitly enable it using the SBI FWFT extension.
>
> IMO, the #2 and #3 are definitely obvious but still worth mentioning.
>
> > > +            1. Neither svade nor svadu in DT: default to svade.
> > > +            2. Only svade in DT: use svade.
> > These two are not relevant to Svadu, I'd leave them out.
> >
> > > +            3. Only svadu in DT: use svadu.
> > Again, statement of the obvious?
> >
> > > +            4. Both svade and svadu in DT: default to svade (Linux can switch to
> > > +               svadu once the SBI FWFT extension is available).
> > Same here as in the Svade entry.
> >
> > Thanks,
> > Conor.
> Regards,
> Anup
>
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