[ofa-general] Re: [PATCH v3 for 2.6.24] IB/ipoib: enable IGMP for userpsace multicast IB apps

[Roland Dreier]

OK, I will merge this for 2.6.24.  However, I still don't really
understand the changelog entry:

 > The kernel IB stack allows (through the RDMA CM) user space multicast applications
 > to interoperate with IP based apps optionally running at a different IP subnet.
 > 
 > To support this inter-op for the case where the receiving party resides at
 > the IB side, there is a need to handle IGMP (reports/queries) else the local
 > IP router would not forward multicast traffic towards the IB network.

So in other words you have a userspace app that joins an IPoIB
multicast group and then it has to do an IP_ADD_MEMBERSHIP socket
option to trigger IGMP messages being sent out, so that traffic gets
routed to it?

 > This patch does a lookup on the database used for multicast reference counting and
 > enhances IPoIB to ignore multicast group which is already handled by user space, all
 > this under a per device policy flag. That is when the policy flag allows it, IPoIB
 > will not join and attach its QP to a multicast group which has an entry on the database.

And then you don't want the kernel IPoIB driver to actually join the
multicast group for the IP multicast group you added with
IP_ADD_MEMBERSHIP?  Why is that exactly -- this is the part I'm
especially hazy on.

 - R.