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

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Date:   Fri, 21 May 2021 21:05:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Florian Weimer <fweimer@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Dave Hansen via Libc-alpha <libc-alpha@...rceware.org>,
        Len Brown <lenb@...nel.org>, 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>,
        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 Fri, May 21 2021 at 18:19, Florian Weimer wrote:
> * Dave Hansen:
>> On 5/21/21 7:44 AM, Florian Weimer wrote:
>>> Why can't userspace look at XCR0 to make the decision?
>>
>> The thing we're trying to avoid is a #NM exception from XFD (the new
>> first-use detection feature) that occurs on the first use of AMX.
>> XCR0 will have XCR0[AMX]=1, even if XFD is "armed" and ready to
>> generate the #NM.
>
> I see.  So essentially the hardware wants to offer transparent
> initialize-on-use, but Linux does not seem to want to implement it this
> way.

The hardware offers an exception which can be used to implement that,
but the hardware does not dictate that usage.

If we'd go that way we lost any control over that resource and I can
demonstrate with AVX512 today what kind of consequences that has with
mixed criticality realtime workloads.

The only solution we have today is to disable AVX512 completely, which
sucks because restricted usage can be benefitial for some of the
computations.

The problem is that the approach of user space in general seems to be
blindly_select_max(AVX). I've seen that in quite some places.

With AMX (and the stuff coming next) we have the chance to do proper
resource control and it would be outright stupid not to take that
opportunity.

Thanks,

        tglx