Re: [Qemu-devel] qemu-kvm upstreaming: Do we need -no-kvm-pit and -no-kvm-pit-reinjection semantics?

[Daniel P. Berrange]

On Fri, Jan 20, 2012 at 12:13:48PM +0100, Jan Kiszka wrote:
> On 2012-01-20 11:25, Daniel P. Berrange wrote:
> > On Fri, Jan 20, 2012 at 11:22:27AM +0100, Jan Kiszka wrote:
> >> On 2012-01-20 11:14, Marcelo Tosatti wrote:
> >>> On Thu, Jan 19, 2012 at 07:01:44PM +0100, Jan Kiszka wrote:
> >>>> On 2012-01-19 18:53, Marcelo Tosatti wrote:
> >>>>>> What problems does it cause, and in which scenarios? Can't they be
> >>>>>> fixed?
> >>>>>
> >>>>> If the guest compensates for lost ticks, and KVM reinjects them, guest
> >>>>> time advances faster then it should, to the extent where NTP fails to
> >>>>> correct it. This is the case with RHEL4.
> >>>>>
> >>>>> But for example v2.4 kernel (or Windows with non-acpi HAL) do not
> >>>>> compensate. In that case you want KVM to reinject.
> >>>>>
> >>>>> I don't know of any other way to fix this.
> >>>>
> >>>> OK, i see. The old unsolved problem of guessing what is being executed.
> >>>>
> >>>> Then the next question is how and where to control this. Conceptually,
> >>>> there should rather be a global switch say "compensate for lost ticks of
> >>>> periodic timers: yes/no" - instead of a per-timer knob. Didn't we
> >>>> discussed something like this before?
> >>>
> >>> I don't see the advantage of a global control versus per device
> >>> control (in fact it lowers flexibility).
> >>
> >> Usability. Users should not have to care about individual tick-based
> >> clocks. They care about "my OS requires lost ticks compensation, yes or 
> >> no".
> > 
> > FYI, at the libvirt level we model policy against individual timers, for
> > example:
> > 
> >   <clock offset="localtime">
> >     <timer name="rtc" tickpolicy="catchup" track="guest"/>
> >     <timer name="pit" tickpolicy="delay"/>
> >   </clock>
> 
> Are the various modes of tickpolicy fully specified somewhere?

There are some (not all that great) docs here:

  http://libvirt.org/formatdomain.html#elementsTime

The meaning of the 4 policies are:

      delay: continue to deliver at normal rate
    catchup: deliver at higher rate to catchup
      merge: ticks merged into 1 single tick
    discard: all missed ticks are discarded


The original design rationale was here, though beware that some things
changed between this design & the actual implementation libvirt has:

  https://www.redhat.com/archives/libvir-list/2010-March/msg00304.html

Regards,
Daniel
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