[openib-general] multicast

[Sean Hefty]

Andrew Friedley wrote:
> I'm trying to understand how the ibverbs multicast API works, but I'm 
> not sure how multicast groups are created.  I understand that 
> ibv_attach_mcast() and ibv_detach_mcast() are used to leave/join a 
> particular multicast group, but IB architecture spec indicates a group 
> must be created first.  How do I do this?
> 
> Actually, is there any multicast sample code anywhere?

Before you can call ibv_attach_mcast(), you must first join the multicast group 
by sending a multicast join request to the SA.  The join request may create the 
multicast group if it doesn't already exist, and the request has the necessary 
information.

You can use the rdma_join_multicast() API in the rdma_cm to join a multicast 
group and attach the group to a QP.  If the group has not already been created, 
it will be created for you, with the group parameters based on that used by 
ipoib.  The rdma_join_multicast() call will invoke ibv_attach_mcast() for you 
once the join operation completes.  There is sample code for joining multicast 
groups using this API in userspace/librdmacm/examples/mckey.c.

If you want more details on multicast group creation, you'll need to see the 
spec, section 12.2.5.17.  Code to create / join a multicast group can be found 
in the kernel cma.c module - see cma_join_ib_multicast().

You may want to see if rdma_join_multicast() meets your needs first.  If it does 
not, please let me know what additional functionality you need.

- Sean