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

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Message-ID: <cd11d9423c8b46ecbf97753f8317c294@AcuMS.aculab.com>
Date:   Fri, 23 Nov 2018 09:34:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>
CC:     "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: 22 November 2018 18:58
...
> Oh, and I just noticed that on x86 we expressly use our old "safe and
> sane" functions: see __inline_memcpy(), and its use in
> __memcpy_{from,to}io().
> 
> So the "falls back to memcpy" was always a red herring. We don't
> actually do that.
> 
> Which explains why things work.

It doesn't explain why I've seen single byte PCIe TLP generated
by memcpy_to/fromio().

I've had to change code to use readq/writeq loops because the
byte accesses are so slow - even when PIO performance should
be 'good enough'.

It might have been changed since last time I tested it.
But I don't remember seeing a commit go by.

	David

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