Re: [PATCH v2] x86: Add support for AVX10 version and vec size in cpu-features

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Fri, Sep 1, 2023 at 1:52 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Fri, Aug 25, 2023 at 12:39 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > On Fri, Aug 25, 2023 at 2:12 AM Florian Weimer <fweimer@redhat.com> wrote:
> > >
> > > * Noah Goldstein via Libc-alpha:
> > >
> > > > +/* AVX10 version information is handled differently from all other CPUID
> > > > +   related logic.  Rather than being encoded in cpuid as discrete booleans, the
> > > > +   AVX10 version is encoded is a full byte that represents the version number
> > > > +   (greater than or equal to 1).  Since the CPUID API is only able to handle
> > > > +   boolean returns, we enumerate `x86_cpu_AVX10_V{1..255}` which can be queried
> > > > +   by the user and have special logic in `x86_cpu_{present,active}`.  This is
> > > > +   unpleasant on our end, but is the only way to make the existing API also
> > > > +   support version queries.  */
> > > > +
> > > > +  /* All 1s should never be a value feature index.  */
> > > > +  x86_cpu_AVX10_V255 = ~0,
> > > > +  x86_cpu_AVX10_V254 = x86_cpu_AVX10_V255 - 1,
> > > > +  x86_cpu_AVX10_V253 = x86_cpu_AVX10_V254 - 1,
> > > > +  x86_cpu_AVX10_V252 = x86_cpu_AVX10_V253 - 1,
> > > > +  x86_cpu_AVX10_V251 = x86_cpu_AVX10_V252 - 1,
> > >
> > > Is this really necessary?  We can define an x86_cpu_avx10_version inline
> > > function that calls __x86_get_cpuid_feature_leaf and extracts the
> > > version byte.  Is it really necessary to support something like this?
> > >
> >
> > I agree it's not the best API. My feelings are two fold on this.
> > 1) The existing API that users know should support the feature, even if its
> > not the best way for users query the logic. Adding support for specific version
> > queries in CPU_FEATURE_{ACTIVE,PRESENT} doesn't prevent us from
> > adding an API that allows for querying the full byte. But only adding the byte
> > might be disruptive to users that expect to be able to do all their querying
> > through the existing API.
> > 2) For a lot of use cases, this is sufficient. Currently we only have
> > two versions
> > and I think the common use case is not "is feature above level X?",
> > but "do I have
> > feature X?". The latter can reasonably be supported with a single query of:
> > CPU_FEATURE_{ACTIVE,PRESENT}(AVX10_<MIN_VERSION_WITH_X>).
> >
> > >   CPU_FEATURE_ACTIVE (AVX10_V2)
> > >
> > > This would be the alternative:
> > >
> > >   x86_cpu_avx10_version () >= 2
> > >
> > We do plan to propose a new byte API in a follow up :)
> >
> > > Let's hope that version 0 means no AVX10. 8-)
> > >
> > versions start at 1.
> >
> >
> >
> > > Thanks,
> > > Florian
> > >
>
> Ping.
> Florian, you okay with this going in?

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