linux-kernel - Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

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Message-ID: <CALCETrV=Q7+D4EZiQDOwqF6O=BfMOxCpLed7=YVUJADayKAJ7Q@mail.gmail.com>
Date:   Fri, 16 Apr 2021 15:14:24 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Len Brown <lenb@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Willy Tarreau <w@....eu>,
        Florian Weimer <fweimer@...hat.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...el.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-abi@...r.kernel.org,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        Rich Felker <dalias@...c.org>, Kyle Huey <me@...ehuey.com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

On Fri, Apr 16, 2021 at 3:11 PM Len Brown <lenb@...nel.org> wrote:
>
> > I get it.  That does not explain why LDMXCSR and VLDMXCSR cause
> > pipelines stalls.
>
> Sorry, I thought this thread was about AMX.
> I don't know the answer to your LDMXCSR and VLDMXCSR question.

My point is that every single major math extension since the original
XMM extensions (SSE, etc) has come with performance gotchas.  Given
Intel's general unwillingness to document the gotchas in hardware that
is actually shipping, I'm sceptical that AMX is as delightfully
gotcha-free as you are making it out to be.

Is there any authoritative guidance at all on what actually happens,
performance-wise, when someone does AMX math?