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

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Message-ID: <93e3b500-5992-a674-18e6-445d1db7b1f0@metux.net>
Date:   Mon, 28 Jun 2021 12:14:02 +0200
From:   "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
To:     Len Brown <lenb@...nel.org>, Florian Weimer <fweimer@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen via Libc-alpha <libc-alpha@...rceware.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Rich Felker <dalias@...c.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Kyle Huey <me@...ehuey.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Keno Fischer <keno@...iacomputing.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Willy Tarreau <w@....eu>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related
 features

On 24.06.21 01:11, Len Brown wrote:
>>    x86 CPU features detection for applications (and AMX)
>>    <https://lore.kernel.org/linux-api/87tulo39ms.fsf@oldenburg.str.redhat.com/>
> 
> FWIW, I didn't receive it, because you excluded
> 
> linux-kernel@...r.kernel.org

me neither :(

Maybe just repost it to LKML ?

You mention the interface *was* designed with cpu features remaining
constant over a process' lifetime. Between the line I'm reading that
this might not be the case anymore.

How could that happen ? Process migration on a different CPU (or perhaps
on a different host) ?

This is gonna be tricky, because this somehow needs to be synchronized
with the application (even if we check the bits before calling some
cpu-specific opcode, which also has some performance cost), there's
still some window where the application might not yet recognize the
change. So either we need some explicit migration points (where app
tells, please let me finish this func first) or transparent emulation.

Damn, how could the cpu designers come up with such weird concepts
in the first place ? :o

--mtx

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Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@...ux.net -- +49-151-27565287