Re: [Xen-devel] Ever increasing time offset for HVM domain / Huge amounts of drift

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Jan 14, 2013 at 01:37:01PM +0000, Phil Evans wrote:
> Hi,
> 
> I am currently running Xen 4.2.1 (this has also happened in 4.2.0 as well).  
> We have been having a major problem with sometimes huge amounts of clock 
> drift in Windows VMs.  Sometimes the clock on a VM could suddenly jump by 
> over a week (usually forwards, however time has been known to go backwards as 
> well).
> 
> Now I don?t profess to know the internals of Xen, however through my 
> investigation I believe I have a degree of knowledge of what could be causing 
> the problem.
> 
> The steps to reproduce this (for me at least), is to simply do a manual NTP 
> sync on a Windows VM.  Upon monitoring the qemu-dm log file for the VM, I see 
> similar to the following:
> 
> Time offset set 489, added offset 480
> Time offset set 436, added offset -53
> Time offset set 496, added offset 60
> Time offset set 494, added offset -2
> Time offset set 554, added offset 60
> Time offset set 565, added offset 11
> Time offset set 606, added offset 41
> Time offset set -1974, added offset -2580
> Time offset set 1626, added offset 3600
> Time offset set 1579, added offset -47
> Time offset set 1639, added offset 60
> 
> It seems to add the same number of seconds to the offset as has passed since 
> the last sync.  The offset just keeps on increasing, eventually resulting in 
> huge numbers equating to days.  Occasionally the offset may jump a bit and go 
> down but the general trend is up.  Although this does not affect the VM 
> immediately, at some point I am guessing it syncs itself with the CMOS clock 
> (which is now a large number of seconds offset from the actual time), 
> resulting in a huge jump in time.  A reboot is a guaranteed way to get the 
> new, incorrect time.
> 
> Although I do not understand all of the underlying code, I presume the 
> correct way this should work is it should be comparing the CMOS time that?s 
> just been set with the hardware clock on the physical machine, resulting in 
> an offset between the two.  This would result in a generally stable number 
> (ideally 0).  Obviously it is incorrect behaviour for the number to keep 
> going up.  To my mind it looks like it may be somehow getting an inaccurate 
> time from the system (in many cases a fixed time rather than an up-to-date 
> current time).
> 
> Does anyone have any light they may be able to shed on this?  Is it possible 
> it could be struggling to get an accurate time from the hardware?  I have 
> checked on several occasions and both the system time and the BIOS clock are 
> spot on.
> 

Please paste the cfgfile of your HVM Windows.

-- Pasi


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