Re: Make QEmu HPET disabled by default for KVM?

[Gleb Natapov <gleb@redhat.com>]

On Thu, Mar 11, 2010 at 10:46:06AM +0200, Avi Kivity wrote:
> On 03/11/2010 10:42 AM, Gleb Natapov wrote:
> >On Thu, Mar 11, 2010 at 04:38:48PM +0800, Sheng Yang wrote:
> >>On Thursday 11 March 2010 16:31:57 Gleb Natapov wrote:
> >>>On Thu, Mar 11, 2010 at 10:28:12AM +0200, Avi Kivity wrote:
> >>>>On 03/11/2010 10:23 AM, Sheng Yang wrote:
> >>>>>>>I have kept --no-hpet in my setup for
> >>>>>>>months...
> >>>>>>Any details about the problems?  HPET is important to some guests.
> >>>>>Seems like HPET reaction is too slow to satisfy some guests(for it would
> >>>>>replace PIT).
> >>>>>
> >>>>>Here is the thread last time.
> >>>>>
> >>>>>http://thread.gmane.org/gmane.comp.emulators.kvm.devel/44899
> >>>>Thanks.  We can address this in three ways: first, adjust the guest
> >>>>not to do timing related tests when virtualized (since no matter
> >>>>what we do, the tests may fail).  Second, I think we should
> >>>>implement userspace ack notifiers (similar to tpr access notifiers
> >>>>already present).  Third, we can implement a kernel hpet, which,
> >>>>after we solve the zillion bug it introduces, will also give a nice
> >>>>performance improvement for hpet intensive workloads.
> >>>Second will not solve the problem. Presence of ack notifiers will not
> >>>make HPET interrupt arrive faster.
> >>The slow may also due to lost tick. And with the lost tick, hpet is still
> >>unusable...
> >>
> >If the problem it due to lost ticks reinjection may solve it, but only partially.
> >What if IO thread haven't run even once during the time vcpu did clock
> >source check? IIRC sometimes we trigger this even with in kernel PIT.
> 
> That is true.  Reinjection can correct problems in the long term,
> but may fail in the short term.  10 ticks is easily short term in a
> heavily loaded system.
> 
> How does it happen with kernel PIT?  I could understand it if we had
> a work item doing the injection, but everything happens either from
> hrtimer context or vcpu context.
> 
Do we kick vcpu out of guest mode when hrtimer triggers? I don't see us doing it in
__kvm_timer_fn().

--
			Gleb.