linux-kernel - Re: [RFC patch 0/4] TSC calibration improvements

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Date:	Thu, 4 Sep 2008 23:33:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Alok Kataria <akataria@...are.com>,
	Arjan van de Veen <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC patch 0/4] TSC calibration improvements

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, 4 Sep 2008, Ingo Molnar wrote:
> > 
> > hm, unless i'm missing something i think here we still have a small 
> > window for an SMI or some virtualization delay to slip in and cause 
> > massive inaccuracy: if the delay happens _after_ the last 
> > pit_expect_msb() and _before_ the external get_cycles() call. Right?
> 
> Yes. I had the extra pit_expect_msb() originally, but decided that 
> basically a single-instruction race for somethign that ran without any 
> MSI for 15ms was a bit pointless.

the race is wider than that i think: all it takes an SMI at the last PIO 
access, so the window should be 1 usec, against a 15000 usecs period. 
That's 1 out of 15,000 boxes coming up with totally incorrect 
calibration.

we also might have a very theoretical race of an SMI taking exactly 65 
msecs so that the whole PIT wraps around and fools the fastpath - the 
chance for that would be around 1:300 - assuming we only have to hit the 
right MSB with a ~200 usecs precision). That assumes equal distribution 
of SMI costs which they certainly dont have - most of them are much less 
than 60 msecs. So i dont think it's an issue in practice - on real hw.

But it's still a possibility unless i'm missing something. We could 
protect against that case by reading the IRQ0-pending bit and making 
sure it's not pending after we have done the closing TSC readout.

> But adding another pit_expect_msb() is certainly not wrong.

ok, i kept that bit.

	Ingo
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