Re: [Xen-devel] Xen timing mode

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxx> wrote:

> 
> A question about timing modes in Xen came up after reading the documentation
> on 
> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=(inter
> 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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