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

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Date:   Thu, 20 May 2021 17:22:58 -0400
From:   Len Brown <lenb@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, 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 API <linux-api@...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>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

On Thu, May 20, 2021 at 4:54 PM Thomas Gleixner <tglx@...utronix.de> wrote:

Thomas,

> > AMX is analogous to the multiplier used by AVX-512.
> > The architectural state must exist on every CPU, including HT siblings.
> > Today, the HT siblings share the same execution unit,
> > and I have no reason to expect that will change.
>
> I'm well aware that HT siblings share the same execution unit for
> AVX.
>
> Though AMX is if I remember the discussions two years ago correctly
> shared by more than the HT siblings which makes things worse.

I regret that we were unable to get together in the last year to have
an updated discussion.  I think if we had, then we would have saved
a lot of mis-understanding and a lot of email!

So let me emphasize here:

There is one TMUL execution unit per core.
It is shared by the HT siblings within that core.

So the comparison to the AVX-512 multiplier is a good one.

Len Brown, Intel Open Source Technology Center