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Date: Fri, 16 Jul 2010 21:05:00 +0000
From: Bruce Cran <brucec@muon.cran.org.uk>
To: Alexander Motin <mav@FreeBSD.org>
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Cc: Rob Farmer <rfarmer@predatorlabs.net>, current <current@freebsd.org>
Subject: Re: Clock not moving in virtual machine
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On Fri, Jul 16, 2010 at 11:47:23PM +0300, Alexander Motin wrote:
> 
> It is probably hard to see pattern due to to very high clock frequency.
> But TSC timecounter is unreliable even on real SMP systems. What it
> counts on virtual SMP - even bigger question. As system seems never uses
> timecounters with negative quality - you've left with
> kern.timecounter.hardware=dummy - that's why time is not going. As last
> resort you may try to set sysctl kern.timecounter.hardware=TSC in run time.

I came across the same problem on rootbsd a few days ago, and set the TSC 
as the timecounter in /etc/sysctl.conf - I've since found it should be 
possible to also set kern.timecounter.smp_tsc=1 in /boot/loader.conf to let 
the TSC be chosen. The system's now been running for a day and I've not had 
any warnings about the clock going backward, and since the time has 
remained correct I guess Xen synchronises with the host? Should I still 
switch back to using the i8254? 

-- 
Bruce Cran