[ofa-general] Re: Understanding IPoIB-CM

[Amar Mudrankit]

On 8/5/07, Michael S. Tsirkin <mst at dev.mellanox.co.il> wrote:
>
> > 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.


    So, there are two connections established between 2 communicating peers.
1 connection  each for transmitting in each direction. Is it correct?

>   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.


        Here, I wanted to ask  - before transmitting why is the skb->dev
field  modified as the device is same IPoIB interface through which packets
will be transmitted. Is p->dev is some other device?

--
> MST
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openfabrics.org/pipermail/general/attachments/20070806/67722ae7/attachment.html>