Re: pv clock: kvm is incompatible with xen :-(

[Gerd Hoffmann <kraxel@redhat.com>]

Jeremy Fitzhardinge wrote:
> Gerd Hoffmann wrote:
>> Wall clock is off a few hours though.  Oops.
>>
>> I think the way wall clock and system clock work together in xen (Jeremy
>> correct me if I'm wrong) is that the wall clock specifies the point in
>> time where the system clock started going.  As kvm fills in host system
>> time into the guest system time fields the guest wall clock fields
>> should be filled with the host boot time timestamp I'd say.
>>   
> 
> Yes.  The wallclock field in the shared info structure is the wallclock
> time at boot; you compute the current time by adding the system
> timestamp to it.   System time changes are effected by retroactively
> changing the boot time of the machine, though that can also change
> because of suspend/resume/migrate.
> 
> In general the kernel only reads the wallclock time at boot, and then
> maintains it for itself from then on.  I think.

Thanks.

I'm looking at the guest side of the issue right now, trying to identify
common code, and while doing so noticed that xen does the
version-check-loop in both get_time_values_from_xen(void) and
xen_clocksource_read(void), and I can't see any obvious reason for that.
 The loop in xen_clocksource_read(void) is not needed IMHO.  Can I drop it?

cheers,
  Gerd

-- 
http://kraxel.fedorapeople.org/xenner/

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