linux-kernel - Re: [PATCH v2 32/36] x86/entry: Micro-optimize compat fast syscall arg fetch

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Date:	Fri, 9 Oct 2015 09:32:06 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2 32/36] x86/entry: Micro-optimize compat fast syscall
 arg fetch


* Andy Lutomirski <luto@...nel.org> wrote:

> we're following a 32-bit pointer, and the uaccess code isn't smart
> enough to figure out that the access_ok check isn't needed.
> 
> This saves about three cycles on a cache-hot fast syscall.

Another request: could you please stick the benchmarking code of the various x86 
system call variants into 'perf bench' - under tools/perf/bench/, so that 
measurements can be done on more hardware and can be reproduced easily?

I'd suggest we dedicate an entirely new benchmark family to it: 'perf bench x86' 
and then have:

   perf bench x86 syscall vdso
   perf bench x86 syscall int80
   perf bench x86 syscall vdso-compat

or so?

( I have some perf bench cleanups in -tip, so if you do this please base it on top 
  of that. )

Thanks,

	Ingo
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