From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Borislav Petkov <bp@alien8.de>,
tglx@linutronix.de, mingo@redhat.com, X86 ML <x86@kernel.org>,
hpa@zytor.com, Andy Lutomirski <luto@kernel.org>,
open list <linux-kernel@vger.kernel.org>,
aubrey <aubrey.li@intel.com>,
"Chen, Tim C" <tim.c.chen@intel.com>,
"Van De Ven, Arjan" <arjan.van.de.ven@intel.com>
Subject: Re: [PATCH v1] x86/fpu: Remove opmask state from avx512_timestamp check
Date: Fri, 15 Oct 2021 12:30:42 -0500 [thread overview]
Message-ID: <CAFUsyfJxASE5MTBU=5eJWt7vXr0fHMBxuyqFJ5T9FEb3Yena=w@mail.gmail.com> (raw)
In-Reply-To: <53c43d2d-0e2e-654b-417d-d3dcbca42fc5@intel.com>
On Fri, Oct 15, 2021 at 10:25 AM Dave Hansen <dave.hansen@intel.com> 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.
next prev parent reply other threads:[~2021-10-15 17:31 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-20 5:39 [PATCH v1] x86/fpu: Remove opmask state from avx512_timestamp check Noah Goldstein
2021-09-27 18:02 ` Noah Goldstein
2021-10-13 22:36 ` Noah Goldstein
2021-10-14 8:28 ` Borislav Petkov
2021-10-14 15:49 ` Noah Goldstein
2021-10-15 14:40 ` Borislav Petkov
2021-10-15 15:25 ` Dave Hansen
2021-10-15 17:30 ` Noah Goldstein [this message]
2021-10-15 20:47 ` [PATCH v2 1/2] x86/fpu: Add helper function for tracking AVX512 status Noah Goldstein
2021-10-26 23:15 ` Noah Goldstein
2021-10-15 20:47 ` [PATCH v2 2/2] x86/xstate: Make AVX512 status tracking more accurate Noah Goldstein
2021-10-27 11:07 ` Borislav Petkov
2021-10-27 16:28 ` Noah Goldstein
2021-10-27 16:58 ` Borislav Petkov
2021-10-27 17:18 ` Noah Goldstein
2021-10-27 16:26 ` [PATCH v3 1/2] x86/fpu: Add helper function for tracking AVX512 status Noah Goldstein
2021-10-27 16:26 ` [PATCH v3 2/2] x86/xstate: Make AVX512 status tracking more accurate Noah Goldstein
2021-10-27 17:11 ` Borislav Petkov
2021-10-27 17:37 ` Noah Goldstein
2021-10-27 17:45 ` Borislav Petkov
2021-10-27 18:21 ` Noah Goldstein
2021-11-03 20:22 ` Noah Goldstein
2021-11-03 20:48 ` Borislav Petkov
2021-11-03 20:56 ` Noah Goldstein
2021-10-27 17:17 ` [PATCH v4 1/2] x86/fpu: Add helper function for tracking AVX512 status Noah Goldstein
2021-10-27 17:17 ` [PATCH v4 2/2] x86/xstate: Make AVX512 status tracking more accurate Noah Goldstein
2021-10-27 18:21 ` [PATCH v5 1/2] x86/fpu: Add helper function for tracking AVX512 status Noah Goldstein
2021-10-27 18:21 ` [PATCH v5 2/2] x86/xstate: Make AVX512 status tracking more accurate Noah Goldstein
2021-11-16 16:21 ` [PATCH] x86/fpu: Correct AVX512 state tracking Borislav Petkov
2021-11-16 16:49 ` Noah Goldstein
2021-11-17 18:29 ` [tip: x86/fpu] " tip-bot2 for Noah Goldstein
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