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From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features
Date:   Thu, 15 Apr 2021 10:38:19 -0700
Message-Id: <C5EFC9AE-3A62-480D-86FD-79B0ADA7E472@amacapital.net>
References: <25f1cca7-fba7-63c1-f11d-17a3069a9ceb@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Len Brown <lenb@kernel.org>,
        Willy Tarreau <w@1wt.eu>, Florian Weimer <fweimer@redhat.com>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        linux-abi@vger.kernel.org, libc-alpha@sourceware.org,
        Rich Felker <dalias@libc.org>, Kyle Huey <me@kylehuey.com>,
        Keno Fischer <keno@juliacomputing.com>
In-Reply-To: <25f1cca7-fba7-63c1-f11d-17a3069a9ceb@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Precedence: bulk


> On Apr 15, 2021, at 10:00 AM, Dave Hansen <dave.hansen@intel.com> wrote:
>=20
> =EF=BB=BFOn 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.
>=20
> 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.

>=20
> 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. :)