Re: [PATCH v6 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

[Jessica Clarke]

On 28 Jun 2024, at 17:19, Conor Dooley <conor@xxxxxxxxxx> wrote:
> 
> On Fri, Jun 28, 2024 at 05:37:06PM +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 | 28 +++++++++++++++++++
>> 1 file changed, 28 insertions(+)
>> 
>> diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
>> index 468c646247aa..c3d053ce7783 100644
>> --- a/Documentation/devicetree/bindings/riscv/extensions.yaml
>> +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
>> @@ -153,6 +153,34 @@ 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 SW-managed PTE A/D
>> +            bit updates 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 present in DT =>
> 
>>                It is technically
>> +               unknown whether the platform uses Svade or Svadu. Supervisor may
>> +               assume Svade to be present and enabled or it can discover based
>> +               on mvendorid, marchid, and mimpid.
> 
> I would just write "for backwards compatibility, if neither Svade nor
> Svadu appear in the devicetree the supervisor may assume Svade to be
> present and enabled". If there are systems that this behaviour causes
> problems for, we can deal with them iff they appear. I don't think
> looking at m*id would be sufficient here anyway, since the firmware can
> have an impact. I'd just drop that part entirely.

Older QEMU falls into that category, as do Bluespec’s soft-cores (which
ours are derived from at Cambridge). I feel that, in reality, one
should be prepared to handle both trapping and atomic updates if
writing an OS that aims to support case 1.

Jess