[ofw] [RFC] getting an RDMA driver interface

[Sean Hefty]

I wanted to get feedback on adding the ability to the low level RDMA driver to
respond to IRP_MN_QUERY_INTERFACE IRPs.  This would return a supported
direct-call verb interface.

This would be used by upper level drivers in the corresponding device stack
(e.g. the WinVerbs filter driver) to retrieve the verb calls.

(Longer term, the IB Bus driver could migrate to be a part of an HCA's device
stack, allowing it to replace the current HCA registration model with standard
PnP add/remove device handling.  It could then use the same query.  However, I'm
not looking at making this change in the near future.)

The verb interface itself would remain unchanged.  The driver would simply
respond to the IRP appropriately.  This has an additional benefit of allowing
different drivers to evolve their interfaces on independent timelines.  (E.g.
ConnectX driver could use a newer interface that supported XRC, while the mthca
driver would continue to use the existing interface.  This would avoid needing
to update a driver because another device exposed additional calls.)

Does anyone see any issues with this approach or have other comments?  It
appears that there would at least need to be a mechanism for an RDMA device to
invoke different callbacks in order to handle multiple drivers above it.
(Alternatively, a verb filter driver could do this, something like a much, much
simpler version of IBAL that only dealt with verb calls directly.)

- Sean