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Date: Mon, 29 Jul 2013 17:27:28 +0200
From: Frederic Konrad <fred.konrad@greensocs.com>
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Subject: Re: [Qemu-devel] [RFC 0/3] Determinitic behaviour with icount.
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, mark.burton@greensocs.com, qemu-devel@nongnu.org

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.

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