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Re: [PATCH] x86/uaccess: use unrolled string copy for short strings

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On Wed, Jun 21, 2017 at 4:09 AM, Paolo Abeni <pabeni@redhat.com> wrote:
>
> +       if (len <= 64)
> +               return copy_user_generic_unrolled(to, from, len);
> +
>         /*
>          * If CPU has ERMS feature, use copy_user_enhanced_fast_string.
>          * Otherwise, if CPU has rep_good feature, use copy_user_generic_string.

NAK. Please do *not* do this. It puts the check in completely the
wrong place for several reasons:

 (a) it puts it in the inlined caller case (which could be ok for
constant sizes, but not in general)

 (b) it uses the copy_user_generic_unrolled() function that will then
just test the size *AGAIN* against small cases.

so it's both bigger than necessary, and stupid.

So if you want to do this optimization, I'd argue that you should just
do it inside the copy_user_enhanced_fast_string() function itself, the
same way we already handle the really small case specially in
copy_user_generic_string().

And do *not* use the unrolled code, which isn't used for small copies
anyway - rewrite the "copy_user_generic_unrolled" function in that
same asm file to have the non-unrolled cases (label "17" and forward)
accessible, so that you don't bother re-testing the size.

                 Linus

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Thread

[PATCH] x86/uaccess: use unrolled string copy for short strings Paolo Abeni <pabeni@redhat.com> - 2017-06-21 13:20 +0200
  Re: [PATCH] x86/uaccess: use unrolled string copy for short strings Kees Cook <keescook@chromium.org> - 2017-06-21 19:40 +0200
    Re: [PATCH] x86/uaccess: use unrolled string copy for short strings Alan Cox <gnomes@lxorguk.ukuu.org.uk> - 2017-06-22 17:00 +0200
  Re: [PATCH] x86/uaccess: use unrolled string copy for short strings Ingo Molnar <mingo@kernel.org> - 2017-06-22 10:50 +0200
    Re: [PATCH] x86/uaccess: use unrolled string copy for short strings Paolo Abeni <pabeni@redhat.com> - 2017-06-22 19:10 +0200
  Re: [PATCH] x86/uaccess: use unrolled string copy for short strings Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-22 19:40 +0200
    Re: [PATCH] x86/uaccess: use unrolled string copy for short strings Paolo Abeni <pabeni@redhat.com> - 2017-06-22 20:00 +0200
    [PATCH] x86/uaccess: optimize copy_user_enhanced_fast_string for short string Paolo Abeni <pabeni@redhat.com> - 2017-06-29 16:00 +0200
      Re: [PATCH] x86/uaccess: optimize copy_user_enhanced_fast_string for  short string Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-29 23:50 +0200
      [tip:x86/asm] x86/uaccess: Optimize  copy_user_enhanced_fast_string() for short strings tip-bot for Paolo Abeni <tipbot@zytor.com> - 2017-06-30 15:20 +0200

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