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Re: [Xen-devel] [PATCH 2/3] timekeeping: sync persistent clock and RTC on system time step changes

>>> On 14.05.13 at 19:15, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On 05/14/2013 02:47 AM, David Vrabel wrote:
>> On 14/05/13 01:40, John Stultz wrote:
>>> I'm sorry, this isn't quite making sense to me. Could you further
>>> describe the exact problematic behavior you're seeing here, and why its
>>> a problem?
>> The Xen wallclock is used as the persistent clock for Xen guests.  This
>> is initialized (by Xen) with the CMOS RTC at the start of day.
> Start of the day? I assume you mean on dom0 bootup? Or is it done 
> pre-dom0 bootup by Xen itself?

It is, indeed - Xen reads the CMOS clock (or consults EFI) once
when it starts up, but leaves those alone as soon as it launched

>>    If the
>> RTC is incorrect then guests will see an incorrect wallclock time until
>> dom0 has corrected it.
> Sorry, just a bit more clarifying context here: So there is a 1:1 
> relationship between xen_wall_clock and the RTC for all domN guests? And 
> even if dom0 has set its system time properly, domN guests will 
> initialize (in effect) from the hardware RTC and not from dom0's system 
> time?

No, (PV) DomU-s will get their time from the software clock that
Xen maintains, which Dom0 is helping keep in good shape when

So the hypervisor really doesn't care about the actual RTC getting
updated in hardware, all it needs is for Dom0 to notify it of wall
clock corrections.


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