Re: Make QEmu HPET disabled by default for KVM?

Dor Laor
2010-03-14T12:50:09Z

On 03/14/2010 12:27 PM, Avi Kivity wrote:
> On 03/14/2010 12:23 PM, Dor Laor wrote:
>> On 03/14/2010 09:10 AM, Gleb Natapov wrote:
>>> On Sun, Mar 14, 2010 at 09:05:50AM +0200, Avi Kivity wrote:
>>>> On 03/11/2010 09:08 PM, Marcelo Tosatti wrote:
>>>>>
>>>>>>
>>>>>>> I have kept --no-hpet in my setup for
>>>>>>> months...
>>>>>> Any details about the problems? HPET is important to some guests.
>>>>> As Gleb mentioned in the other thread, reinjection will introduce
>>>>> another set of problems.
>>>>>
>>>>> Ideally all this timer related problems should be fixed by correlating
>>>>> timer interrupts and time source reads.
>>>>
>>>> This still needs reinjection (or slewing of the timer frequency).
>>>> Correlation doesn't fix drift.
>>>>
>>> But only when all time sources are synchronised and correlated with
>>> interrupts we can slew time frequency without guest noticing (and only
>>> if guest disables NTP)
>>
>> In the mean time we should definitely disable hpet by default.
>
> Definitely not. Windows needs it. Some pre-kvmclock Linux may also work
> with it.
>
> Without hpet, there is no fast high resolution timer in the system.

It's all depends on how hard would it be to re-inject to windows guest.
We still need to fix the win2k3 64 bit and win2k8 64 bit (and not win7
as I told initially) since the irq is broadcasted to all the vcpus and
we do not track who acknowledged the irq.

>
>> Besides this we need to fully virtualize the tsc, fix win7 64bit rtc
>> time drift and some pvclock potential issues. Before we add new timer,
>> better fix existing ones.
>>
>> What about creating a pv time keeping device that will be aware of
>> lost ticks and host wall clock time? It's similar to hyper-v
>> enlightenment virt timers.
>
> That's kvmclock.
>

I meant a device that can be used to generate timeouts. We do use today
pit/rtc along with kvmclock time source but it's not perfect and
probably the same for hpet. This is why I tough that a pv device will be
beneficial.