Re: [PATCH] replay: improve determinism of virtio-net

[Alex Bennée]

Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru> writes:

> On 31.05.2021 07:55, Jason Wang wrote:
>> 在 2021/5/17 下午9:04, Pavel Dovgalyuk 写道:
<snip>
>>> @@ -2546,7 +2547,7 @@ static void
>>> virtio_net_handle_tx_bh(VirtIODevice *vdev, VirtQueue *vq)
>>>           return;
>>>       }
>>>       virtio_queue_set_notification(vq, 0);
>>> -    qemu_bh_schedule(q->tx_bh);
>>> +    replay_bh_schedule_event(q->tx_bh);
>> Not familiar with replay but any chance to change qemu_bh_schedule()
>> instead?
>
> It would be better, but sometimes qemu_bh_schedule is used for the
> callbacks that are not related to the guest state change.

Maybe that indicates we should expand the API:

  void qemu_bh_schedule(QEMUBH *bh, bool guest_state);

or maybe explicit functions:

  void qemu_bh_schedule(QEMUBH *bh);
  void qemu_bh_schedule_guest_update(QEMUBH *bh);

And document the cases with proper prototypes in main-loop.h.

While I was poking around I also saw qemu_bh_schedule_idle which made me
wonder what happens if a guest change is triggered by this. Would this
be impossible to make deterministic because we don't know when the bh
actually get activated?

My concern with just adding replay_bh_schedule_event in the appropriate
places is we end up with a patchwork of support for different devices
and make it very easy to break things.

-- 
Alex Bennée