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[Xen-devel] Clock jumped 50 minutes in dom0 caused incorrect 2008 R2 dom

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Subject: [Xen-devel] Clock jumped 50 minutes in dom0 caused incorrect 2008 R2 domU time
From: Mark Adams <mark@xxxxxxxxxxxxxxxxxx>
Date: Wed, 6 Oct 2010 12:16:18 +0100
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Hi Xen-Devel's

Please see my note below regarding a serious issue where my clock jumped
in dom0. I'm sending this through to the devel list as I haven't managed
to glean any clear help from xen-users and the debian bug team are
unsure what could have caused this.

Can you confirm if the kernel or xen controls the clock in dom0? I also
understand that this could be an underlying hardware issue but I have
another system on exactly the same hardware which hasn't had this occur.

Any advice on how to investigate further or ensure better clock
stability across dom0 and domU would be appreciated. 

Also is it correct behaviour for Xen to reboot an 2008 R2 HVM domU if
the time moves this much? My guess is that the domU crashed when the
time changed, and was thus rebooted automatically. Strangely the Windows
2003 server didn't get rebooted.

If you need any more info to help please let me know.


On Mon, Oct 04, 2010 at 01:00:51PM +0100, Mark Adams wrote:
> On Mon, Oct 04, 2010 at 11:01:10AM +0100, Mark Adams wrote:
> > Hi All,
> > 
> > Im running Xen 4.0.1-rc6 Debian squeeze with pvops 2.6.32-21 kernel.
> > Today I noticed (when kerberos to the domain controllers stopped
> > working..) that the clock was 50 minutes out in dom0 -- This caused the
> > HVM windows domain controllers to have the wrong time.
> > 
> > I'm not sure if this is a kernel issue or a xen issue, but the only
> > thing related is I can see the following in the kernel log:
> > 
> > Oct  2 18:50:33 havhost1 kernel: [623480.977748] Clocksource tsc unstable 
> > (delta = -2999660303788 ns)
> > 
> > But I also see in the dmesg log that xen is using it's own clock.
> > 
> > [    7.676563] Switching to clocksource xen
> > 
> > I can't identify anything else in the logs to indicate when the time
> > might have changed. I have a few other dom0 at the same level that
> > haven't decided to change the time.
> > 
> > Can anyone confirm whether xen controls the time or the kernel? Also
> > when I corrected the time in dom0 it was still wrong in HVM domU -- How
> > long does it take for this to propogate? (I rebooted the VM's to correct
> > it immediately).
> > 
> > Any other pointers on how to ensure stability of clocks from dom0 to
> > domU HVM hosts (and pv for that matter..) would be appreciated.
> Some further info on this, It appears the HVM domU (windows server 2008)
> unexpectedly shut down at 18:51, after the unstable clocksource error.
> qemu-dm logs show a reset "reset requested in cpu_handle_ioreq." and
> xend.log shows a reboot 
> [2010-10-02 18:51:03 1759] INFO (XendDomainInfo:2088) Domain has shutdown: 
> name=ha-dc1 id=2 reason=reboot.
> This is like someone issuing "xm reboot domain" is it not? Is it
> possible that xen could have issued this reboot itself due to a crash? I
> can't see any crash logs.
> Cheers,
> Mark

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