Date: Fri, 21 Jun 2024 10:37:21 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Anup Patel <apatel@...tanamicro.com>, Conor Dooley <conor@...nel.org>
Cc: 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, ajones@...tanamicro.com, 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 20/06/2024 08:25, 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.
>
>>> +            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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