Re: [PATCH] x86: Don't set Prefer_No_AVX512 for processors with AVX512 and AVX-VNNI

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Dec 6, 2021 at 11:47 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Sun, Dec 5, 2021 at 9:23 PM H.J. Lu via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > Don't set Prefer_No_AVX512 on processors with AVX512 and AVX-VNNI since
> > they won't lower CPU frequency when ZMM load and store instructions are
> > used.
> > ---
> >  sysdeps/x86/cpu-features.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
> > index be2498b2e7..311ade1f26 100644
> > --- a/sysdeps/x86/cpu-features.c
> > +++ b/sysdeps/x86/cpu-features.c
> > @@ -538,8 +538,11 @@ init_cpu_features (struct cpu_features *cpu_features)
> >           |= bit_arch_Prefer_No_VZEROUPPER;
> >        else
> >         {
> > -         cpu_features->preferred[index_arch_Prefer_No_AVX512]
> > -           |= bit_arch_Prefer_No_AVX512;
> > +         /* Processors with AVX512 and AVX-VNNI won't lower CPU frequency
> > +            when ZMM load and store instructions are used.  */
> > +         if (!CPU_FEATURES_CPU_P (cpu_features, AVX_VNNI))
> > +           cpu_features->preferred[index_arch_Prefer_No_AVX512]
> > +             |= bit_arch_Prefer_No_AVX512;
> >
> >           /* Avoid RTM abort triggered by VZEROUPPER inside a
> >              transactionally executing RTM region.  */
> > --
> > 2.33.1
> >
>
> Should we also do Rocket Lake?
> According to Travis Downs at least downclocking is an issue there ether:
> https://travisdowns.github.io/blog/2020/08/19/icl-avx512-freq.html#rocket-lake

Thiago, Arjan,

Is this true that Rocket Lake can use ZMM load/store?

-- 
H.J.