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Bug#599161: xen-linux-system-2.6.32-5-xen-amd64: Clock moved forward 50 minutes, caused Xen HVM domU restart



Hi Ben, Thanks for your response. See my responses inline.

On Wed, Oct 06, 2010 at 03:35:23AM +0100, Ben Hutchings wrote:
> On Tue, 2010-10-05 at 09:31 +0100, Mark Adams wrote:
> > Package: xen-linux-system-2.6.32-5-xen-amd64
> > Version: 2.6.32-21
> > Severity: important
> > 
> > 
> > Hi, Did you receive this bug report? I hadn't received a bug ID even
> > though receiving the copy of the original report.
> 
> We don't have any other bug report with this description.
> 
> > Likely because the address it was sent from originally was invalid.
> 
> That would explain it.
> 
> > -----------------
> > 
> > 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.                                                                                            
> 
> Since you appear to be in the UK, is it possible that the real-time
> clock is set to local time (GMT+1) while Xen expects it to be GMT, or
> vice versa?

The clock is set with tzdata as BST yes, it is also set to this in the
Windows server 2008 domU. We are using localtime=1 to match the clock
in dom0 to domU.

> 
> (This doesn't explain why it's 50 minutes out rather than 1 hour.  But
> ntpd will refuse to correct a large difference and the local clock may
> then drift further.)
> 
> [...]
> > 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).                                                                                                                                  
> [...]
> 
> For HVM guests, the hypervisor emulates a standard PC real-time clock
> and the guest uses that to initialise the system time, but there is no
> way to force an update after the guest has booted unless the guest has
> specific support for Xen; I assume Citrix does provide such software for
> Windows but I don't know whether it is free software.

The citrix WHQL drivers might have this functionality, I don't use them
though - prefer the GPL PV drivers! (which don't have any clock support
as far as I can tell)

> 
> For PV guests, I assume you can force an update to the guest time using
> the Xen management tools.
> 
> Note, I'm just a general kernel maintainer and don't have any great
> knowledge of Xen.

All good, I have a feeling it might be a kernel issue rather than xen,
but I'm still not sure what actually -controls- the time, is it the
kernel? I think the key is in the log

Oct  2 18:50:33 havhost1 kernel: [623480.977748] Clocksource tsc unstable (delta = -2999660303788 ns)
> 
As this is when the clock went from 18:00 to 18:50 and started the chain
of events (restarted the 2008 domU). Any ideas why this log occurred?

> Ben.

Regards,
Mark
> 
> -- 
> Ben Hutchings
> Once a job is fouled up, anything done to improve it makes it worse.






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