[openib-general] RFC multicast support

[Roland Dreier]

 > 1. I'd like to expand the CMA to include an asynchronous rdma_set_option()
 > routine.
 > 
 > 2. This routine would become the user interface to joining and leaving multicast
 > groups.  For example:
 > 
 > rdma_set_option(struct rdma_cm_id *, IPPROTO_IP, IP_ADD_MEMBERSHIP,
 > 		    struct in_addr *, sizeof(struct in_addr));

I'm not sure this makes sense.  Only IB has the notion of multicast on
QPs and only UD QPs can be used for multicast.  So why would we want a
transport-neutral API for an IB-specific feature?

 > 4. Join/leave requests would be tracked by the local SA cache.  A port would not
 > be removed from the group while there were active members.  Optionally, a port
 > could remain in the group without any members for some user specified duration.
 > (I'm not sure how useful this would be in practice.)

Reference counting multicast group joins is definitely something that
IB needs.  Otherwise there's no way for userspace to use IB multicast
in a sane way.

 - R.