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

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Message-ID: <7ea44458b90b4d41a08ba9012818d273@AcuMS.aculab.com>
Date:   Thu, 22 Nov 2018 17:36:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Denys Vlasenko' <dvlasenk@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, "Ingo Molnar" <mingo@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Brian Gerst <brgerst@...il.com>,
        "linux-kernel@...r.kernel.org" <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: Denys Vlasenko
> Sent: 21 November 2018 13:44
...
> I also tested this while working for string ops code in musl.
> 
> I think at least 128 bytes would be the minimum where "REP insn"
> are more efficient. In my testing, it's more like 256 bytes...

What happens for misaligned copies?
I had a feeling that the ERMS 'reb movsb' code used some kind
of barrel shifter in that case.

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.
Byte reads on PCIe are really horrid.

	David

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