Re: [PATCH 07/13] IB: add a proper completion queue abstraction

[Sagi Grimberg]

> > If we have not drained the CQ (consumed budget or more) the second
> > condition (ib_req_notify_cq) will not be invoked. We are only rearming
> > the CQ when we drained it completely. So I don't see how we can end up
> > with missed notifications.
> We drain the CQ completely for whatever CQEs available at that time,
> say for example,
> 33 CQEs drained at time t1. So now req_notify_cq will be invoked at time t2.
> During time delta t2-t1, CQ in hardware remains unarmed.
> If cqes are added during that time delta, Will event/interrupt raised
> for it, for CQ in unarmed state?
>
> At time time t2, CQ is armed containing pending CQEs. Will
> event/interrupt raised for those pending CQEs on next arming?

If the device did not reported missed-events then it is the device
responsibility to generate a new completion event after arming.

Specifically, the mlx4/mlx5 HW is able to generate a completion event
in your described scenario. A device that is not capable of doing so
must report missed events to inform the core it has more completions
to consume.