[openib-general] Re: [PATCH 2/6] [RFC] mthca kernel changes for resizeCQ

[Michael S. Tsirkin]

Quoting r. Roland Dreier <rdreier at cisco.com>:
> I'm not sure I got it right but it looks OK to me.
> 
> I don't really see any other way to do this, because the RESIZE_CQ
> command can block, so we can't lock out poll CQ operations while the
> firmware command is executing.

I thought about this some more. Its unfortunate that there's an overhead
on each poll cq operation.

We could avoid this overhead by:
- adding poll_cq callback and lock into the struct ib_cq,
  initializing them on cq creation.
- modifying ib_poll_cq to

static inline int ib_poll_cq(struct ib_cq *cq, int num_entries,
                             struct ib_wc *wc)
{
	unsigned long flags;
	int ret;
	spin_lock_irqsave(cq->lock, flags);
        ret = cq->poll_cq(cq, num_entries, wc);
	spin_unlock_irqrestore(cq->lock, flags);
	return ret;
}

Now, device driver can modify the poll_cq callback in the cq
to a slower version that checks two buffers for the duration of the resize
operation.

How does this sound?

-- 
Michael S. Tsirkin
Staff Engineer, Mellanox Technologies