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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 23 Nov 2018 09:42:39 -0800
Message-ID: <CAHk-=wjrdrZW5VgnKXqdayafYmQC2HBEA4kzMWYTyaj5jgURdA@mail.gmail.com>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes
To:     David.Laight@aculab.com
Cc:     Andrew Lutomirski <luto@kernel.org>, dvlasenk@redhat.com,
        Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, bp@alien8.de,
        Peter Anvin <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, brgerst@gmail.com,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        pabeni@redhat.com
Content-Type: multipart/mixed; boundary="0000000000007a8721057b5883b2"
Sender: linux-kernel-owner@vger.kernel.org
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--0000000000007a8721057b5883b2
Content-Type: text/plain; charset="UTF-8"

On Fri, Nov 23, 2018 at 8:36 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Let me write a generic routine in lib/iomap_copy.c (which already does
> the "user specifies chunk size" cases), and hook it up for x86.

Something like this?

ENTIRELY UNTESTED! It might not compile. Seriously. And if it does
compile, it might not work.

And this doesn't actually do the memset_io() function at all, just the
memcpy ones.

Finally, it's worth noting that on x86, we have this:

  /*
   * override generic version in lib/iomap_copy.c
   */
  ENTRY(__iowrite32_copy)
          movl %edx,%ecx
          rep movsd
          ret
  ENDPROC(__iowrite32_copy)

because back in 2006, we did this:

    [PATCH] Add faster __iowrite32_copy routine for x86_64

    This assembly version is measurably faster than the generic version in
    lib/iomap_copy.c.

which actually implies that "rep movsd" is faster than doing
__raw_writel() by hand.

So it is possible that this should all be arch-specific code rather
than that butt-ugly "generic" code I wrote in this patch.

End result: I'm not really all that  happy about this patch, but it's
perhaps worth testing, and it's definitely worth discussing. Because
our current memcpy_{to,from}io() is truly broken garbage.

                   Linus

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