Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/07/2011 08:58 AM, Avi Kivity wrote:
> On 02/07/2011 04:54 PM, Anthony Liguori wrote:
>>
>>>   Why the accumulated_ticks argument?
>>
>> Then the missing ticks is stored in the PeriodicTimer instead of 
>> storing it in the device state.  That means we won't forget to save 
>> it in vmstate.
>>
>> It's convenient because then if we lose ticks in the PeriodicTimer 
>> layer, the devices have instance access to that info.  When you do a 
>> read() from timerfd, it returns the number of coalesced events.  
>> That's the interface I had in my mind.
>>
>> We could just add a getter for PeriodicTimer and it would serve the 
>> same purpose.
>
> If a drift compensation policy is in effect, you don't need the missed 
> ticks, since you will get one callback for each (delayed) tick.  If 
> there is no drift compensation policy, presumably you aren't 
> interested in lost ticks.  So the ticks argument isn't very useful.
>
> On the other hand, we need a way to inject lost ticks into a 
> PeriodicTimer.  If interrupt injection detects that an interrupt was 
> coalesced, we want the timer to schedule a new tick for us.

As an optimization, if you can do something useful with the knowledge 
that we missed 20 ticks instead of waiting for 20 callbacks, it's useful 
to have.  But adding a getter makes that possible and I agree that it 
clutters the interface for an edge use-case.

Regards,

Anthony Liguori