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

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Date:   Mon, 26 Nov 2018 10:26:08 +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: 23 November 2018 16:36
...
> End result: we *used* to do this right. For the last eight years our
> "memcpy_{to,from}io()" has been entirely broken, and apparently even
> the people who noticed oddities like David, never reported it as
> breakage but instead just worked around it in drivers.

I've mentioned it several times...

Probably no one else noticed lots of single byte transfers while
testing a TLP monitor he was writing for an FPGA :-)
They are far too expensive to buy, and would never be connected
to the right system at the right time - so we (I) wrote one.

Unfortunately we don't really get to see what happens when the
link comes up (or rather doesn't come up). We only get the
LTSSM state transitions.

	David

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