[Xen-users] Timing Adjustment on Xen

[Priya <pbhat@xxxxxxxxxxxx>]

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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