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

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Date:   Mon, 7 Jan 2019 09:55:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>
CC:     Andrew Lutomirski <luto@...nel.org>,
        "dvlasenk@...hat.com" <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" <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" <brgerst@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>
Subject: RE: [PATCH] x86: only use ERMS for user copies for larger sizes

From: Linus Torvalds
> Sent: 05 January 2019 02:39
...
> Anyway, it would be lovely to hear whether memcpy_toio() now works
> reasonably. I just picked our very old legacy function for this, so it
> will do things in 32-bit chunks (even on x86-64), and I'm certainly
> open to somebody doing something smarter, but considering that nobody
> else seemed to show any interest in this at all, I just went
> "whatever, good enough".
> 
> I tried to make it easy to improve on things if people want to.

I'll do some tests once the merge has had time to settle.

I needed to open-code one part because it wants to do copy_to_user()
from a PCIe address buffer (which has to work).

Using 64bit chunks for reads is probably worth while on x86-64.
I might cook up a patch.
Actually, if the AVX registers are available without an fpu save
using larger chunks would be worthwhile - especially for io reads.

	David

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