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Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAHk-=wgpmV2B=s2xE5385Jzy2tEZuiW+MSPveJBfpqUoFdGydg@mail.gmail.com>
Date:   Fri, 23 Nov 2018 12:11:19 -0700
Cc:     David.Laight@aculab.com, Andrew Lutomirski <luto@kernel.org>,
        dvlasenk@redhat.com, Jens Axboe <axboe@kernel.dk>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, bp@alien8.de,
        Peter Anvin <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, brgerst@gmail.com,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        pabeni@redhat.com
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References: <02bfc577-32a5-66be-64bf-d476b7d447d2@kernel.dk> <20181121063609.GA109082@gmail.com> <48e27a3a-2bb2-ff41-3512-8aeb3fd59e57@kernel.dk> <26eff539-7de7-784c-0c88-f1d30753299d@redhat.com> <7ea44458b90b4d41a08ba9012818d273@AcuMS.aculab.com> <CAHk-=wjLNuNpHRfuVTb6awstC9OOmSb3S8aP51ufep-Th4dXKw@mail.gmail.com> <CALCETrW=hMZLUM_nPEEgUW9x9Bmhm2LZ6ZRDiRRguxVT=-_BWA@mail.gmail.com> <CAHk-=wiEUVpSU-GxpKDvN9wnH2yDxkJgE7Vqh46A+fmnvhubyw@mail.gmail.com> <cd11d9423c8b46ecbf97753f8317c294@AcuMS.aculab.com> <64fd67993af04579b5262c270a7a4694@AcuMS.aculab.com> <CAHk-=wj9DA-mH8jahuEPUWxO4EgbCcPRgdOJAQD58DBtCCrHcQ@mail.gmail.com> <CAHk-=wjrdrZW5VgnKXqdayafYmQC2HBEA4kzMWYTyaj5jgURdA@mail.gmail.com> <C74CD745-0E6D-410C-B942-416AF365B492@amacapital.net> <CAHk-=wgpmV2B=s2xE5385Jzy2tEZuiW+MSPveJBfpqUoFdGydg@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 23, 2018, at 11:44 AM, Linus Torvalds <torvalds@linux-foundation.or=
g> wrote:
>=20
>> On Fri, Nov 23, 2018 at 10:39 AM Andy Lutomirski <luto@amacapital.net> wr=
ote:
>>=20
>> What is memcpy_to_io even supposed to do?  I=E2=80=99m guessing it=E2=80=99=
s defined as something like =E2=80=9Ccopy this data to IO space using at mos=
t long-sized writes, all aligned, and writing each byte exactly once, in ord=
er.=E2=80=9D  That sounds... dubiously useful.
>=20
> We've got hundreds of users of it, so it's fairly common..
>=20

I=E2=80=99m wondering if the =E2=80=9Cat most long-sizes=E2=80=9D restrictio=
n matters, especially given that we=E2=80=99re apparently accessing some of t=
he same bytes more than once. I would believe that trying to encourage 16-by=
te writes (with AVX, ugh) or 64-byte writes (with MOVDIR64B) would be safe a=
nd could meaningfully speed up some workloads.

>> I could see a function that writes to aligned memory in specified-sized c=
hunks.
>=20
> We have that. It's called "__iowrite{32,64}_copy()". It has very few users=
.
>=20
>                  Linus