[RFC] bulk zero copy transport

[Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>]

Just wanted to float an idea we are thinking about. It builds on the basic idea
of what Intel submitted as their RV module [1]. This however does things a bit
differently and is really all about bulk zero-copy using the kernel. It is a new
ULP.

The major differences are that there will be no new cdev needed. We will make
use of the existing HFI1 cdev where an FD is needed. We also propose to make use
of IO-Uring (hence needing FD) to get requests into the kernel. The idea will be
to not share Uverbs objects with the kernel. The kernel will maintain
ownership of the qp, pd, mr, cq, etc.

Connections we envision to be maintained by the kernel using RDMA CM. Similar in
fashion to how RDS or IPoIB works. This of course means an RC QP which allows
our TID RDMA feature to work under the hood.

We have looked into RDS and RTRS and both seem to be the wrong interface. RDS
provides a lot of what we are looking for but it seems to be a bit overkill and
has higher overhead than we hope to achieve. Performance results show it to be
less performant than direct to verbs.

After reviewing the RV submission, I don't think there is any reason to try to
revamp that submission. It seems to be very tightly tied to PSM3 whereas this is
meant to be more generic.

At this point we are interested in what questions you would have or opinions. We
would like to get some feedback early in the process. As we develop the code
we'll continue to post, similar to how we did rdmavt and welcome anyone that
wants to collaborate.

[1] https://lore.kernel.org/linux-rdma/20210319125635.34492-1-kaike.wan@intel.com/

-Denny