[PATCH v5 2/4] dt-bindings: riscv: Add Svade and Svadu Entries
Alexandre Ghiti
alex at ghiti.fr
Fri Jun 21 01:37:21 PDT 2024
On 20/06/2024 08:25, Anup Patel wrote:
> On Wed, Jun 5, 2024 at 10:25 PM Conor Dooley <conor at kernel.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 at sifive.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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