Re: [Qemu-devel] [RFC 0/3] Determinitic behaviour with icount.

[Frederic Konrad <fred.konrad@greensocs.com>]

On 29/07/2013 18:42, Paolo Bonzini wrote:
> Il 29/07/2013 17:27, Frederic Konrad ha scritto:
>> On 18/07/2013 17:35, Paolo Bonzini wrote:
>>> Il 18/07/2013 17:06, Peter Maydell ha scritto:
>>>> On 18 July 2013 16:02,<fred.konrad@greensocs.com>  wrote:
>>>>> As I said in the last email, we have issues with determinism with
>>>>> icount.
>>>>> We are wondering if determinism is really ensured with icount?
>>>> My opinion is that it *should* be deterministic but it would
>>>> be unsurprising if the determinism had got broken along the way.
>>> First of all, it can only be deterministic if the guest satisfies (at
>>> least) all the following condition:
>>>
>>> 1) only uses timer that QEMU bases on vm_clock (which means that you
>>> should use "-rtc clock=vm"---sorry Fred, didn't think about this in the
>>> previous answer);
>>>
>>> 2) never does any network operation nor any asynchronous disk I/O
>>> operation
>>>
>>> 3) never halts the VCPU waiting for an interrupt
>> Hi,
>>
>> qemu_alarm is making the replay not deterministic too.
> What is qemu_alarm?  If you mean qemu_alarm_timer, then that means
> rt_clock and host_clock (item 1 above)?
>
> If so, yes, I believe you need to record/replay them.  When doing replay
> for reverse execution, you certainly want to execute at full speed
> without waiting for real time to pass again.
>
> Paolo

Yes, it was what we believed too. :)

Thanks,
Fred

>> We tried to remove those alarms and it seems to replay well (at least
>> far better).
>>
>> So the question is: how we can solve that?
>>
>> We thought at two possibilities :
>>    * record/replay them, like IO.
>>    * base them on our new ic_clock.
>>
>> Both have drawbacks:
>>    * record/replay won't make icount more deterministic (run to run).
>>    * ic_clock speed time is apparently not constant.
>>
>> Thanks,
>> Fred
>>