Re: [Qemu-devel] QEMU and Real Time OS

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

On 30/01/2015 11:26, Marc Marí wrote:
> El Fri, 30 Jan 2015 08:37:47 +0100
> Jan Kiszka <jan.kiszka@siemens.com> escribió:
>> On 2015-01-30 00:06, Paolo Bonzini wrote:
>>>
>>> On 29/01/2015 20:37, Marc Marí wrote:
>>>> Is this an expected behaviour? I can't see why.
>>>>
>>>> I'd like to know if there is a certain reason why it doesn't work.
>>>> Or if it should work and the problem is too much I/O overhead. Or
>>>> any other hint to understand it.
>>> It is due to latencies in the host.  You need at least to use
>>> preempt-rt kernels in the host as well.
>> That alone won't help much. You also need to fine-tune the guest to
>> avoid running into QEMU locks that continuously synchronizes the guest
>> on things like VGA or disk I/O emulation.
>>
>> When using KVM, thus being able to run VCPUs widely independent of
>> each other and the device models, you need to push cyclictest on an
>> isolated second virtual CPU of the guest. Luiz and Marcelo can
>> probably confirm this based on their ongoing experiments.
>>
>> With TCG, we would first of all have to make it true SMP and
>> independent of the I/O device lock. That's what Frederic is working
>> on [1].
>>
>> Jan
>>
>> [1] http://permalink.gmane.org/gmane.comp.emulators.qemu/314406
>>
> Thanks for the answers. I think I'm stuck with ARM926, which I think is
> not prepared for SMP. I'll have to look if I can use Cortex for my
> experiments.
>
> I'll continue interested with the improvements for RT on TCG, but for
> the moment I'll go to work on real harware, even though is easier to
> run and debug on an emulator.
>
> Thanks
> Marc
Hi Marc,

I think the important point here is "TCG thread independent of the I/O 
device lock".
I need it for multithread TCG but that doesn't mean you need an SMP guest
platform for that.

Fred