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Message-ID: <1498154074.2503.11.camel@redhat.com>
Subject: Re: [PATCH] x86/uaccess: use unrolled string copy for short strings
From: Paolo Abeni <pabeni@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: the arch/x86 maintainers <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date: Thu, 22 Jun 2017 19:54:34 +0200
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         <CA+55aFzLF6dvHS_N9-M-=6J83HZ3rp+JJ68NSpKc+D_yBA=o_w@mail.gmail.com>
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On Thu, 2017-06-22 at 10:30 -0700, Linus Torvalds wrote:
> 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.

Thank you for the feedback.

I'm quite new to the core x86 land; the rep stosb cost popped out while
messing with the networking. I'll try to dig into the asm.

Regards,

Paolo