linux-kernel - Re: [PATCH 00/12] x86: Rewrite 64-bit syscall code

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Date:	Mon, 7 Dec 2015 14:55:27 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/12] x86: Rewrite 64-bit syscall code

On Mon, Dec 7, 2015 at 1:51 PM, Andy Lutomirski <luto@...nel.org> wrote:
> This is kind of like the 32-bit and compat code, except that I
> preserved the fast path this time.  I was unable to measure any
> significant performance change on my laptop in the fast path.
>
> What do you all think?

For completeness, if I zap the fast path entirely (see attached), I
lose 20 cycles (148 cycles vs 128 cycles) on Skylake.  Switching
between movq and pushq for stack setup makes no difference whatsoever,
interestingly.  I haven't tried to figure out exactly where those 20
cycles go.

--Andy

View attachment "zap_fastpatch.patch" of type "text/x-patch" (2878 bytes)