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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-07 16:08, Avi Kivity wrote:
> On 02/07/2011 05:01 PM, Jan Kiszka wrote:
>>>
>>>  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.
>>
>> Isn't absence of corresponding call to periodic_timer_ack() sufficient?
> 
> It probably is.  However, that API is easy to misuse; if you forget to 
> call it, the timer goes crazy.  The default behaviour should be to 
> assume an ack and the API should provide adjustments.

Explicit nack'ing only works smoothly if we have immediate (synchronous)
feedback about the injected event. My feeling is that ack'ing results in
simpler, thus also easier to review code.

> 
> Also need to design the API carefully for changing frequency (Windows is 
> known to do that) and switching from periodic to single shot.  For the 
> first case I guess we need to adjust the deferred ticks to the new time 
> base (so if the frequency doubles, the lost ticks up to that point 
> double as well).  For the second case, I guess we just lose time.

I think this is rather a question how the logic behind
periodic_timer_mod works with updates. The API should be sufficient as
simple as it is.

Jan

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