Re: Make QEmu HPET disabled by default for KVM?

[Avi Kivity <avi@redhat.com>]

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.

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