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

From: Paolo Abeni
Date: Thu Jun 22 2017 - 13:54:43 EST

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.