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

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Message-Id: <C5EFC9AE-3A62-480D-86FD-79B0ADA7E472@amacapital.net>
Date:   Thu, 15 Apr 2021 10:38:19 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Len Brown <lenb@...nel.org>,
        Willy Tarreau <w@....eu>, Florian Weimer <fweimer@...hat.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-abi@...r.kernel.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 Apr 15, 2021, at 10:00 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 4/15/21 9:24 AM, Andy Lutomirski wrote:
>> In the patches, *as submitted*, if you trip the XFD #NM *once* and you
>> are the only thread on the system to do so, you will eat the cost of a
>> WRMSR on every subsequent context switch.
> 
> I think you're saying: If a thread trips XFD #NM *once*, every switch to
> and from *that* thread will incur the WRMSR cost.

Indeed.  My sentence was missing a few words at the end.

> 
> The first time I read this, I thought you were saying that all threads
> would incur a WRMSR cost on every context switch.  If that's the case, I
> grossly misread the patches. :)