Clock not moving in virtual machine

Bruce Cran brucec at
Fri Jul 16 21:05:05 UTC 2010

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

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