FreeBSD does support Infiniband: https://wiki.freebsd.org/InfiniBand
This page mentions that Mellanox has provided work on this. Also storage vendors uses external infiniband stacks on FreeBSD for many years (I got my first Isilon cluster something like 8 years ago, and it was using an IB backend, with a forked FreeBSD 7 kernel iirc) and are stable and widely deployed.