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

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 09:59 +0100 on 16 Sep (1284631199), Paul Durrant wrote:
> As I understand it wc_sec + wc_nsec should give the number of
> nanoseconds since the Unix epoch at which the system was booted.

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. 

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.

Tim.

>   Paul
> 
> > -----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
> > 
> > Hi
> > 
> > When the virtual machine is resumed from suspend, the guest
> > operating
> > system's wall-clock time remains at the value it had at the time of
> > the
> > suspension. For linux pvm, we can avoid resolve issue by setting
> > independent_wallclock=0. However, HVM(or PVHVM) does not support
> > this
> > feature.
> > 
> > I did some investigation on how to implement this in windows
> > para-virtualization driver. In \include\xen\interface\xen.h, there
> > are
> > 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
> > always
> > zero for my windows vm with para-virtualization driver.
> > Vcpu_time_info
> > only contains time info for specific vcpu. Should i use those
> > variables
> > to get accurate time for VM? Is there any corresponding source code
> > or
> > algorithms available now? Is it safe to simply update vm time after
> > resuming?
> > 
> > Citrix 5.6 Windows para-virtualization driver will notify windows
> > kernel
> > time change after resuming, and resumed windows vm with Citrix pv
> > driver
> > 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.
> > 
> > Thanks
> > Annie
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

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-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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