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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC 0/3] Determinitic behaviour with icount.
Date: Mon, 29 Jul 2013 18:42:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7

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.


> 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

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