Re: Seeking Guidance: Creating an IBV Multicast Group?

[Leon Romanovsky <leon@kernel.org>]

On Mon, Jul 29, 2024 at 01:10:29PM -0400, Andrew Sheinberg wrote:
> Hello all,
> 
> I’m not sure if this the right place to ask, but I will give it a try.
> 
> I have a system with many initialized UD queue pairs (info for address handle creations and qp numbers exchanged out-of-band). I am only using libibverbs for establishment (purposefully not using librdmacm, to allow for more flexible environment configuration) — everything is working smoothly for unicast.  Now I would like to create a multicast group and attach some of these queue pairs (ibv_mcast_attach); however I am struggling to find any details on how to create such a group (and obtain a proper MGID and MLID). 

First call to ibv_attach_mcast() will create a new multicast group.

> 
> I found a few examples online but am left with questions:
> 	- There is code within perftest's "multicast_resources.c", but this seems a bit hacky and oddly verbose
> 	- There is code within Nvidia Docs’  "Programming Examples using IBV” showcasing joining an already created multicast group at a given IP address using rdma_cm, but It is unclear how to create the group in the first place
> 
> 
> Questions (please correct me if these do not make sense):
> 
> 1. What is the role of the OpenSM — is there a C API?

SM stands for subnet management and its goal to oversee fabric topology
and manage path routing among the nodes.

Thanks