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

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Message-ID: <1498154074.2503.11.camel@redhat.com>
Date:   Thu, 22 Jun 2017 19:54:34 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/uaccess: use unrolled string copy for short strings

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