Is there any reason for this driver not supporting LSO or CSUM offloads? I know this is a virtual netdev but I think it should support those options and enabled them depending on the PMD to send or receive packets from.
About giving to the KNI that info, maybe using the kernel firmware interface could be an option. I could work on this and send a RFC with more details, but I would like to know first if there is any limitations or any other thing I should be aware of.