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Re: [Xen-devel] how to keep time of windows pvhvm synchronized with host

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] how to keep time of windows pvhvm synchronized with host after resuming
From: ANNIE LI <annie.li@xxxxxxxxxx>
Date: Fri, 17 Sep 2010 18:44:58 +0800
Cc: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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More specifically, they're what you should add to the current Xen system
time to get the wallclock time (they can change over time as the system
time drifts).  I'm surprised to hear that they're zero.  Even if dom0
isn't updating them (which IIRC pv-ops dom0 doesn't) they should have
been set from the RTC at boot time.  It's possible that your domain's
shared-info page is in the wrong word size - if you haven't set up a
hypercall page or set HVM_PARAM_CALLBACK_IRQ since the domain was
created (i.e. since restore) it might be wrong. 
Maybe this is not a problem of shared-info page.
The problem is wc_sec and wc_nsec does not change until i modify wallclock time of windows vm manually.
If i do not change the wallclock, wc_sec will keep the same value. For example: the initial value of wc_sec is zero,
then it changes to 0x4c64da92 if i change wallclock, 0x4c64da92 is about 40 years since 1970.

I am using xen3.4 for test, any patch i missed?

One problem with using wc_[n]sec to get the wallclock time is that the
usual Xen way to get the system time is to interpolate it from the RDTSC
value and the per-vcpu time info, but in a HVM guest the OS may have
changed the TSC offset to something that the driver can't figure out.
The Citrix PV drivers use the HVMOP_get_time hypercall to get the system
time instead; that hypercall is in xen-unstable but not the 4.0 branch.

If you're using a linux guest with Stefano's pv-on-hvm patches then I
think you should be able to read the PV system time as normal.



-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of ANNIE LI
Sent: 16 September 2010 09:31
To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] how to keep time of windows pvhvm synchronized
with host after resuming


When the virtual machine is resumed from suspend, the guest
system's wall-clock time remains at the value it had at the time of
suspension. For linux pvm, we can avoid resolve issue by setting
independent_wallclock=0. However, HVM(or PVHVM) does not support

I did some investigation on how to implement this in windows
para-virtualization driver. In \include\xen\interface\xen.h, there
several variables about timer such as: wc_sec and wc_nsec, and
vcpu_time_info struct. It is very strange, wc_sec and wc_nsec is
zero for my windows vm with para-virtualization driver.
only contains time info for specific vcpu. Should i use those
to get accurate time for VM? Is there any corresponding source code
algorithms available now? Is it safe to simply update vm time after

Citrix 5.6 Windows para-virtualization driver will notify windows
time change after resuming, and resumed windows vm with Citrix pv
can keep it's time synchronized with host. Would you like to give me
some clues about how to implement this?

Any help is greatly appreciated.


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