linux-kernel - Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a configurable timer

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Message-ID: <20150609184348.GJ22105@pd.tnic>
Date:	Tue, 9 Jun 2015 20:43:48 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Aaron Lu <aaron.lu@...el.com>, Tony Li <tony.li@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Len Brown <lenb@...nel.org>, Ken Xue <ken.xue@....com>,
	Huang Rui <ray.huang@....com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a
 configurable timer

On Tue, Jun 09, 2015 at 10:55:15AM -0700, Andy Lutomirski wrote:
> When I looked at the rdtsc ordering a couple years ago, I thought
> about what it meant for rdtsc to be properly ordered.  I decided that
> proper rdtsc ordering meant that no one should ever be able to tell if
> rdtsc ends up reordered.  Concretely, I think that rdtsc should be
> ordered like an x86 load from a shared memory location.  The manuals
> are vague but, after a decent amount of experimentation,
> rdtsc_barrier(); rdtsc() seems to achieve that on all CPUs.  With the
> barrier, the rdtsc won't happen before a prior load in the same
> thread, and no CPU seems to ever execute rdtsc after a subsequent
> memory access.

That sounds weak to me. I think we will need some enlightenment from hw
people here before we go assume stuff.

> > By virtue of the address dependency?
> 
> No, it's just that CPUs seem to work this way.

Err, that sounds funny. And it must be the data dependency forcing the
RDTSC to execute in order in that case.

-- 
Regards/Gruss,
    Boris.

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