Re: Xen timing mode

[Priya <pbhat@acis.ufl.edu>]

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Thanks !

In that case, how does a para-virtualized user domain find the domain -
virtual time ?

--Priya

On Tue, Feb 23, 2010 at 12:08 PM, Keir Fraser <keir.fraser@eu.citrix.com>wrote:

> Independent_wallclock is specific to Linux PV guests, and is set within the
> guest. Timer_mode is specific to HVM guests and is a domain build-time
> config option.
>
>  -- Keir
>
> On 23/02/2010 16:50, "PBhat" <pbhat@acis.ufl.edu> wrote:
>
> >
> > A question about timing modes in Xen came up after reading the
> documentation
> > on
> >
> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=(inter<http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28inter>
> > rupts)|(timer)
> > 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 '
> http://openskill.info/infobox.php?ID=1451
> > independent wallclock ' to 'dependent clock' and vice versa. The way to
> do
> > this is to toggle the parameter /proc/sys/xen/independent_wallclock
> between
> > 0 (dependent) and 1 (independent)
> >
> >
> >     * 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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