Re: Make QEmu HPET disabled by default for KVM?

Avi Kivity
2010-03-14T10:27:52Z

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.

> 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.

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