Re: [Xen-users] Timing Adjustment on Xen

[Christopher Chen <muffaleta@xxxxxxxxx>]

I've found that having the DomU's time locked to the Dom0 leads to
weird confusion after a live migration, even when both Dom0 machines
are locked to the same NTP host. I've cut them loose in my linux DomUs
with independent wallclock.

Sorry, I'm sure that's not what you wanted to know, but an observation.

On Mon, Mar 16, 2009 at 6:17 PM, Priya <pbhat@xxxxxxxxxxxx> wrote:
> Hello all,
>
> I have some questions about how timing is taken care of on different
> Domains in Xen.
>
> 1. How is time kept in Dom0? Considering that dom0 is also a virtual
> machine, one possibility is that its timing interface might also be
> virtualized. However it could be counting the actual number of
> interrupts sent by the physical (not virtualized) timer. I am not sure
> which one these is the case.
>
> 2. The Xen documentation
> (http://www.linuxtopia.org/online_books/linux_virtualization/xen_3.0_interface_guide/linux_virualization_xen_interface_6.html)
> says that
>
> Xen includes a periodic ticker which sends a timer event to the
> currently executing domain every 10ms. The Xen scheduler also sends a
> timer event whenever a domain is scheduled; this allows the guest OS
> to *adjust*  for the time that has passed while it has been inactive.
> In addition, Xen allows each domain to request that they receive a
> timer event sent at a specified system time by using the
> set_timer_op() hypercall. Guest OSes may use this timer to implement
> timeout values when they block.
>
> My question is, how exactly does a guest domain "adjust" for the
> difference between its time and the actual time. In other words, what
> mechanisms are used to compensate for the lost time during which the
> guest dom was not in context?
>
> 3. I need to access the Time Stamp counter (this is what it is called
> in VMWare) from DomU. In other words, I need to know how many ticks of
> the physical hardware counter passes between two instructions getting
> executed on a user/guest domain. What is the way to do this?
>
> Cheers!
> ./pr
>
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>



-- 
Chris Chen <muffaleta@xxxxxxxxx>
"I want the kind of six pack you can't drink."
-- Micah

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