[Xen-users] Xen Timing Mode

[PBhat <pbhat@xxxxxxxxxxxx>]

A question about timing modes in Xen came up after 
http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions reading the
documentation on configuration file options  and I was wondering if you
could help me with it.

    * I know that Xen has a facility on the user domains to change the
timing from something called the 'independent_wallclock' to
'dependent_wallclock' and vice versa. The way to do this is to toggle the
parameter /proc/sys/xen/independent_wallclock between 0 (dependent) and 1
(independent).  http://openskill.info/infobox.php?ID=1451 Reference 


    * Now, the documentation on the configuration options for xen virtual
machine under the heading of Timers contains no mention of the above
mentioned facility, but instead talks about something called a 'timer_mode' 


 timer_mode: Timer mode (0=delay virtual time when ticks are missed;
1=virtual time is always wallclock time
      timer_mode (default=1; Value='TIMER_MODE')

Now my question is whether the timer mode  = 1 is equivalent to independent_
wallclock?

I think the answer is no. While timer_mode = 1 decides whether the virtual
machine is able to read the Domain-0 time, the independent_wallclock decides
whether you need NTP sychronization on both (user and control) domains or
not.

Namely, in the timer_mode = 1, the gettimeofday() called from the virtual
machine will return the domain virtual time, but time_mode = 0 
gettimeofday() instruction from the virtual machine will return the system
time.

The independent and dependent modes have an effect on whether the virtual
machines can use the NTP or any other synchronization of Domain-0.

Is my understanding correct?

I further want to know whether the working remains similar on Xen HVMs also.
In the sense that are Hardware based virtual machines capable of reading the
Domain-0 system time in an analogous way?


Your help would be much appreciated. I am new to this area and I apologize
if my questions seem too basic.
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