Re: Make QEmu HPET disabled by default for KVM?

[Gleb Natapov <gleb@redhat.com>]

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.

--
			Gleb.