linux-kernel - Re: [Xen-devel] [patch 14/33] xen: xen time implementation

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Date:	Wed, 06 Jun 2007 10:56:34 +0100
From:	Keir Fraser <keir@...source.com>
To:	Jan Beulich <jbeulich@...ell.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Xen-devel <xen-devel@...ts.xensource.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, lkml <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	<virtualization@...ts.osdl.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [patch 14/33] xen: xen time implementation

On 6/6/07 10:30, "Jan Beulich" <jbeulich@...ell.com> wrote:

>> If you have an ACPI PM timer in your system (and if you have SMM then your
>> system is almost certainly modern enough to have one) then surely the
>> problem is fixed for all practical purposes? The problem was overflow of a
>> fixed-width platform counter. The PIT wraps every ~50ms, but the ACPI PM
>> timer will wrap only every ~4s. It would be quite unreasonable for SMM to
>> take the CPU away for multiple seconds, even as a one-time boot operation.
> 
> No, I don't think the problem's gone with the PM timer - it is just much less
> likely. Since you depend on the TSC (which must generally be assumed be
> unsyncronized across CPUs) and on the error correction factor (which shows
> non-zero values every few seconds), getting the interpolated times on two
> CPUs out of sync is still possible, and given the way the time keeping code
> works even being off by just a single nanosecond may be fatal.

If the error across CPUS is +/- just a few microseconds at worst then having
the clocksource clamp to no less than the last timestamp returned seems a
reasonable fix. Time won't 'stop' for longer than the cross-CPU error, and
that should always be a tiny value.

 -- Keir

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