Re: [PATCH v1] x86/fpu: Remove opmask state from avx512_timestamp check

[Noah Goldstein]

On Fri, Oct 15, 2021 at 10:25 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 10/14/21 8:49 AM, Noah Goldstein wrote:
> > Irrelevant of the still existing flaws, it makes the output more accurate.
> >
> > Is there a cost to the change I am not seeing?
>
> We'd want to make sure that this doesn't break anything.  It probably
> won't, but it theoretically could.
>
> For instance, if someone was doing:
>
>         avx512_foo();
>         xsave->xstate_bv &= ~XFEATURE_MASK_ZMMS;
>         XRSTOR(xsave, -1);
>
> That would leave the opmask in place, but would lead to the ZMM
> registers tracked as being in their init state.

The 'XFEATURE_MASK_ZMMS' is new to this patch so I don't think
this patch could be adding that issue.

>
> This would be *very* unlikely, but it would be great if Aubrey (the
> original avx512_timestamp patch author) could make sure that it doesn't
> break anything.
>
> Also, there's the side issue of AVX-256 use.  AVX-256 uses the ZMM
> registers which are a part of XFEATURE_MASK_AVX512, but does not incur
> the same frequency penalties of the full 512-bit-wide instructions.
> Since ZMM_Hi256 is the *only* ZMM state which is truly 512-bit-only, we
> could argue that it's the only one we should consider.
>
> Noah, thanks for bringing this up.  I'm not opposed to your patch, but
> let's just make sure that it doesn't break anything and also that we
> shouldn't do a bit more at the same time (ignore Hi16_ZMM for
> avx512_timestamp).

I think that may make sense. Or outputting separate timestamps for
both. Especially because in GLIBC we have moved to preferring
EVEX implementings for all x86_64 string functions because
vzeroupper aborts RTM transactions:
https://sourceware.org/bugzilla/show_bug.cgi?id=27457

So if an application is using GLIBC on an avx512 machine most likely
the avx512 indicator will be permanently set.