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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

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?

(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.

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.


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

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