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

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Message-ID: <CAHk-=wjLNuNpHRfuVTb6awstC9OOmSb3S8aP51ufep-Th4dXKw@mail.gmail.com>
Date:   Thu, 22 Nov 2018 09:52:40 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David.Laight@...lab.com
Cc:     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>,
        Andrew Lutomirski <luto@...nel.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 Thu, Nov 22, 2018 at 9:36 AM David Laight <David.Laight@...lab.com> wrote:
>
> The other problem with the ERMS copy is that it gets used
> for copy_to/from_io() - and the 'rep movsb' on uncached
> locations has to do byte copies.

Ugh. I thought we changed that *long* ago, because even our non-ERMS
copy is broken for PCI (it does overlapping stores for the small tail
cases).

But looking at "memcpy_{from,to}io()", I don't see x86 overriding it
with anything better.

I suspect nobody uses those functions for anything critical any more.
The fbcon people have their own copy functions, iirc.

But we definitely should fix this. *NONE* of the regular memcpy
functions actually work right for PCI space any more, and haven't for
a long time.

                 Linus