Thank you for posting your question on the Mellanox Community.
Based on your information provided (We also noticed your posting on https://mailman.stanford.edu/pipermail/mininet-discuss/2018-August/008031.html), currently we do hot have support for running Soft-RoCE on a Mininet topology. Currently we only provided a method of running from a nic-2-nic.
Also please note that Soft-RoCE is still in BETA
We would recommend to post this question on the Mininet Mailings-list as you already did. Maybe somebody from the mailings-list has a solution for this as they need to implement this into the Mininet Framework
Thanks and regards,
~Mellanox Technical Support