[PATCH v1] riscv: cpufeature: Add Zic64b ISA extension support

[Qingwei Hu]

Hi Vivian,

Thanks for the feedback.

> On May 29, 2026, at 11:14, Vivian Wang <wangruikang at iscas.ac.cn> wrote:
> 
> On 5/28/26 22:16, Qingwei Hu wrote:
>> Zic64b describes a platform with 64-byte cache block sizes. It is a
>> mandatory part of RVA23U64, but a system may provide 64-byte CMO block
>> sizes without satisfying all profile requirements.
>> 
>> The ISA string parser currently ignores Zic64b because the extension is
>> not present in the cpufeature table. Add an ISA extension ID and table
>> entry for Zic64b so it can be reported in /proc/cpuinfo when firmware
>> advertises it.
> 
> I'd like to understand: What is the motivation to doing this, given that
> the very same information is already available as riscv,cbo*-block-size
> and hwprobe?

The motivation here is not to replace riscv,cbo*-block-size or hwprobe.
Those are still the right interfaces for software that needs the actual
CBO block size.

This is more about preserving the ISA extension name reported by
firmware. Some platforms report zic64b in the ISA string, but Linux
currently drops it because there is no cpufeature entry, so cpuinfo no
longer matches what firmware advertised. That makes profile/ISA-string
comparison tools a bit awkward.

So the patch only makes zic64b visible as a named ISA extension when
firmware reports it, while still validating it against any CBO block
sizes that firmware provides.

> If an extension is what we want to go with, then
> Documentation/devicetree/bindings/riscv/extensions.yaml also needs
> updating to match.

And yes, if we keep this direction, I agree extensions.yaml should be
updated in v2.

Thanks,
Qingwei Hu