linux-kernel - Re: [PATCH] x86: only use ERMS for user copies for larger sizes

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Message-Id: <0D9E579C-F980-4BB7-A282-42206F6636F3@amacapital.net>
Date:   Fri, 23 Nov 2018 12:11:19 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David.Laight@...lab.com, Andrew Lutomirski <luto@...nel.org>,
        dvlasenk@...hat.com, Jens Axboe <axboe@...nel.dk>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        Peter Anvin <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>, brgerst@...il.com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        pabeni@...hat.com
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes



> On Nov 23, 2018, at 11:44 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Fri, Nov 23, 2018 at 10:39 AM Andy Lutomirski <luto@...capital.net> wrote:
>> 
>> What is memcpy_to_io even supposed to do?  I’m guessing it’s defined as something like “copy this data to IO space using at most long-sized writes, all aligned, and writing each byte exactly once, in order.”  That sounds... dubiously useful.
> 
> We've got hundreds of users of it, so it's fairly common..
> 

I’m wondering if the “at most long-sizes” restriction matters, especially given that we’re apparently accessing some of the same bytes more than once. I would believe that trying to encourage 16-byte writes (with AVX, ugh) or 64-byte writes (with MOVDIR64B) would be safe and could meaningfully speed up some workloads.

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