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

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Message-ID: <CAHk-=wjweUiVewcnk39mUccEN-SjxHeTZdB3VOXZrvvW4Cs3dw@mail.gmail.com>
Date:   Wed, 21 Nov 2018 10:16:57 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     pabeni@...hat.com
Cc:     Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>, dvlasenk@...hat.com,
        brgerst@...il.com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes

On Wed, Nov 21, 2018 at 9:27 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> It would be interesting to know exactly which copy it is that matters
> so much...  *inlining* the erms case might show that nicely in
> profiles.

Side note: the fact that Jens' patch (which I don't like in that form)
allegedly shrunk the resulting kernel binary would seem to indicate
that there's a *lot* of compile-time constant-sized memcpy calls that
we are missing, and that fall back to copy_user_generic().

It might be interesting to just change raw_copy_to/from_user() to
handle a lot more cases (in particular, handle cases where 'size' is
8-byte aligned). The special cases we *do* have may not be the right
ones (the 10-byte case in particular looks odd).

For example, instead of having a "if constant size is 8 bytes, do one
get/put_user()" case, we might have a "if constant size is < 64 just
unroll it into get/put_user()" calls.

                  Linus