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

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

On 18/07/2013 18:35, Paolo Bonzini wrote:
> Il 18/07/2013 18:31, 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);
>> Oops sorry, I didn't mentioned that, but we used rtc clock=vm for our
>> tests.
>>> 2) never does any network operation nor any asynchronous disk I/O
>>> operation
>>>
>>> 3) never halts the VCPU waiting for an interrupt
>>>
>>>
>>> Point 1 is obvious.
>>>
>>>
>>> To explain points 2, let's consider what happens if a block device uses
>>> synchronous vs. asynchronous I/O.
>>>
>>> With synchronous I/O, each block device operation will complete
>>> immediately.  All clocks are stalled during the operation.
>>>
>>> With asynchronous I/O, each block device operation will be done while
>>> the CPU is running.  If the CPU is polling a completion flag, the number
>>> of instructions executed (thus icount) depends on how long it takes to
>>> do I/O.
>> So I suppose this can happen even if there are any network card or block
>> device.
>>
>> We probably need to disable it until we finally save and replay IO, to
>> get this thing working.
> Are you aware of the work that was done on fault tolerance (Kemari)?
> Orit is working on resurrecting it.
>
> Paolo

No, but I will take a look that can be really usefull for IO.

Thanks,
Fred