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

[Frederic Konrad]

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,<address@hidden>  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