RE: [Xen-devel] Domain-Virtual time

[Dan Magenheimer <dan.magenheimer@oracle.com>]

Should be "true" system time, i.e. should be very close to what
you see on a "wallclock" (clock on the wall).

HVM's are sadly very widely varied in the parameters needed
to minimize time drift.  In general in the past, timer_mode=0
(or timer_mode unspecified) would be best for 32-bit Linux
domains, timer_mode=1 would be best for Windows domains,
and timer_mode=2 would be best for 64-bit Linux domains.
However, for best results on Linux, this must be combined with
kernel boot parameters that properly select a clock -- and
on some Linux kernel versions, the parameters needed are
different between 32-bit and 64-bit versions of the same
kernel version.  It is up to providers of HVM templates
(aka "appliances") to choose parameters wisely.

Also, you haven't specified your Xen version, but I believe
Xen 4.0 switches the timer_mode default from 0 to 1 so, sadly,
clock behavior may change when moving an unchanged HVM
domain from pre-4.0 to 4.0.

So for best results you should run ntpd in any Linux HVM
domain (and I don't know what you do in Windows).  But
even ntpd may be inadequate to avoid drift if poor parameters
are chosen.

==========

From: Priya [mailto:pbhat@acis.ufl.edu] 
Sent: Thursday, February 25, 2010 9:04 AM
To: xen-devel@lists.xensource.com; xen-users@lists.xensource.com
Subject: [Xen-devel] Domain-Virtual time

Sorry for multiple emails. I sent the last one from the wrong address.

Can anyone please tell me if the value returned by a time querying instruction like gettimeofday() on a Xen (Linux) HVM is the true (System) time or the Domain-virtual time?

PS: Domain virtual time is defined as the time that progresses at the same pace as cycle counter, but only while a domain is executing. It stops while the domain is de-scheduled where as System time accurately reflects the passage of real time.

I am facing issues because my HVMs show a time drift. 

Thanks