[ofa-general] Re: Understanding IPoIB-CM

[Michael S. Tsirkin]

> Quoting Amar Mudrankit <amar.mudrankit at gmail.com>:
> Subject: Understanding IPoIB-CM
> 
> 
> I have been looking at the IPoIB CM code and have some queries. I'd greatly
> appreciate it if some one could shed some light on it.
> 
> Please correct me if I am wrong, but based on my understanding of the code:
> 
> 1) From functions ipoib_cm_create_tx_qp and ipoib_cm_create_rx_qp, it looks
> like for communicating with a peer, we use one QP for purely
>    doing TX and one QP for purely doing RX (created on receipt of a CM REQ).
> That means for every peer we are connected to, we use two
>   QPs. Why not use a single QP for both TX and RX of data ? On receipt of a REQ
> from a peer or before trying to send data to peer,  we
>   could possibly check if we already have a QP connection to the peer (by
> maintaining and looking up a QP and peer UD QPN + GID
>  mapping) and then just use it. Or is it not as simple as that ?

Sure, this could work. One'd have to be sure to cover all error
cases though.

>   On a related note, the IPoIB CM RFC 4755, section 3.3 covers the case of
> simultaneous IB connection where REQs can cross paths.
>   But the current implementation does not seem to handle this case. Is it
> because we create two QPs anyway and do not need to handle
>   this special case ?

Yes.

> 2] In the ipoib_cm_rep_handler, the skbs seem to be dequeued and queued again.
> I am referring to the code below. What does this exactly 
>    achieve ?
> 
>         skb_queue_head_init(&skqueue);
>         spin_lock_irq(&priv->lock);
>         set_bit(IPOIB_FLAG_OPER_UP, &p->flags);
>         if (p->neigh)
>                 while ((skb = __skb_dequeue(&p->neigh->queue)))
>                         __skb_queue_tail(&skqueue, skb);
>         spin_unlock_irq(&priv->lock);
>         while ((skb = __skb_dequeue(&skqueue))) {
>                 skb->dev = p->dev;
>                 if (dev_queue_xmit(skb))
>                         ipoib_warn(priv, "dev_queue_xmit failed "
>                                    "to requeue packet\n");
>         }

This transmits all skbs.

-- 
MST