linux-kernel - Re: [PATCH v1] x86/fpu: Remove opmask state from avx512

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Message-ID: <YWfqEIaCKOwSWGHF@zn.tnic>
Date:   Thu, 14 Oct 2021 10:28:00 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Noah Goldstein <goldstein.w.n@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, X86 ML <x86@...nel.org>,
        hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] x86/fpu: Remove opmask state from avx512_timestamp
 check

On Wed, Oct 13, 2021 at 05:36:14PM -0500, Noah Goldstein wrote:
> Ping2

Why?

The original patch which added this abomination:

2f7726f95557 ("x86/fpu: Track AVX-512 usage of tasks")

says already:

 the tracking mechanism is imprecise and can theoretically miss AVX-512
 usage during context switch. But it has been measured to be precise
 enough to be useful under real-world workloads like tensorflow and
 linpack.

 If higher precision is required, suggest user space tools to use the
 PMU-based mechanisms in combination.

and as you've noticed, the high 16 regs would cause a false positive
too.

So what is the actual real-life use case for this?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette