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

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Date: Fri, 21 Jun 2024 10:33:03 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Anup Patel <apatel@...tanamicro.com>
Cc: Conor Dooley <conor@...nel.org>, 
	Yong-Xuan Wang <yongxuan.wang@...ive.com>, linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org, alex@...ti.fr, greentime.hu@...ive.com, 
	vincent.chen@...ive.com, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, devicetree@...r.kernel.org
Subject: Re: [PATCH v5 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

On Thu, Jun 20, 2024 at 11:55:44AM GMT, Anup Patel wrote:
> On Wed, Jun 5, 2024 at 10:25 PM Conor Dooley <conor@...nel.org> 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@...ive.com>
> > > ---
> > >  .../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.

This is a good point, and maybe it's worth integrating something that
states this case is technically unknown into the final text. (Even though
historically this has been assumed to mean svade.)

> 
> >
> > > +            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 ?

I'm mostly just +1'ing everything below, but with a minor wording change
suggestion

> 1) Both Svade and Svadu not present in DT => Supervisor may

Neither Svade nor Svadu present...

>     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)
> 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.

Thanks,
drew