> It’s obviously proprietary: it’s non-standard and specific to a
  > single vendor.

That's not how people typically use "proprietary" when referring to open-source code developed collectively by multiple vendors, universities, and thousands of independent contributors.

  > What is the whole idea, though? [...] this particular implementation
  > uses io_uring instead of something non-proprietary like RDMA, is just
  > an implementation detail.

When performance matters, sometimes implementation details are the whole idea.

According to the patch's author at <https://lwn.net/Articles/904638/>, ublk has about twice the throughput of NBD.

> It’s obviously proprietary: it’s non-standard and specific to a single vendor.

I just re-read the LWN article and there's no suggestion of any such thing. Where are you seeing it?

> Serving things to kernel from userland is decades old and commonly used with both NFS and iSCSI.

...and regular FC/SCSI too. I worked on that for four years. Yawn. What's your point?

> The fact that this particular implementation uses io_uring instead of something non-proprietary like RDMA

RDMA implementations are even more likely to be bound up with proprietary bits than vanilla-TCP implementations. RDMA over IB or other even lesser known interconnects (both open/standard and proprietary) existed long before ROCE. Do you even know what "open" vs. "proprietary" mean? There were open network-disk protocols before iSCSI. I worked on such as early as 1989, and developed my own (along with my own user-space block device for Linux and Windows) in 2000. It seems like you've only been exposed to a small set of open/standards based technologies, and deride anything else as "proprietary" even though that's far from accurate. Is there some undisclosed interest at play here?

> is just an implementation detail

It's a very important detail, considering the performance difference.