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

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Message-ID: <20210419215809.GJ9093@zn.tnic>
Date:   Mon, 19 Apr 2021 23:58:09 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Len Brown <lenb@...nel.org>
Cc:     Willy Tarreau <w@....eu>, Andy Lutomirski <luto@...nel.org>,
        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 Mon, Apr 19, 2021 at 05:33:03PM -0400, Len Brown wrote:
> For this to happen, every thread would not only have to include/link-with
> code that uses AMX, but that code would have to *run*.

It looks like either I'm not expressing myself clearly enough or you're
not reading my text: the *library* does that decision automatically!

Which means *every* possible thread on the system.

Which means, *every* thread has a fat 8K buffer attached to it because
the library uses AMX on its behalf by *default*.

> I'm sure that the AI guys are super excited about matrix multiplication,
> but I have a hard time imagining why grep(1) would find a use for it.

It doesn't matter if you're imagining it or not - what matters is if the
decision whether the thread uses AMX or not is put in the hands of the
thread and *NOT* in the hands of the library.

Which means, majority of the threads should not allow AMX and only a
handful who do, will have to explicitly state that. And the library will
have to comply. Not the library decides for every thread itself because
the feature's there.

> Indeed, if anyone expected AMX to be used by every task, we would have
> never gone to the trouble of inventing the XFD hardware to support the
> kernel's lazy 8KB buffer allocation.

If it gives me fat-buffers-off-by-default and on only for a handful
of threads which really want it and *request* it *explicitly*, sure,
whatever gets the job done.

-- 
Regards/Gruss,
    Boris.

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