linux-kernel - Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Message-ID: <20150427194512.GK28871@pd.tnic>
Date:	Mon, 27 Apr 2015 21:45:12 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Mon, Apr 27, 2015 at 09:21:34PM +0200, Denys Vlasenko wrote:
> On 04/27/2015 09:11 PM, Borislav Petkov wrote:
> > A:     709.528485252 seconds time elapsed                                          ( +-  0.02% )
> > B:     708.976557288 seconds time elapsed                                          ( +-  0.04% )
> > C:     709.312844791 seconds time elapsed                                          ( +-  0.02% )
> > D:     709.400050112 seconds time elapsed                                          ( +-  0.01% )
> > E:     708.914562508 seconds time elapsed                                          ( +-  0.06% )
> > F:     709.602255085 seconds time elapsed                                          ( +-  0.02% )
> 
> That's about 0.2% variance. Very small.

Right, I'm doubtful this is the right workload for this. And actually
if even any workload would show any serious difference. Perhaps it all
doesn't really matter and we shouldn't do anything at all.

> Sounds obvious, but. Did you try running a test several times?

All runs so far are done with perf state ... --repeat 10 so, 10 kernel
builds and results are averaged.

> Maybe you are measuring random noise.

Yeah. Last exercise tomorrow. Let's see what those numbers would look
like.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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