There are couple of interesting question:

- backpressure

- what if both userspace and SOCKMAP are doing read() on a socket?

- can one use SOCKMAP to splice only a selected amount of data, and pick up the rest with read()

- What are the parser/verdict program semantics. What can they do.

- what is the SK_MSG abstraction, and how to benefit from it.

and more... some of this is discussed:

http://vger.kernel.org/lpc_net2018_talks/ktls_bpf_paper.pdf

On io_submit, we wrote about it here:

https://blog.cloudflare.com/io_submit-the-epoll-alternative-...

TLDR: it allows for batching, and with IOCB_CMD_POLL can be used as epoll alternative.

Thanks for all the articles! I read the one about io_submit in the meantime. I actually hoped it would be as async as the API suggests, so it seems a bit disappointing for my use-case. Do you know if when I would schedule multiple writes via io_submit, whether it would block until all sockets have written something, or only until at least one has written something?

I guess the fact that one has to guess and try how those APIs behave makes me most nervous and would make me prevent from using those. An async IO system should be very well-behaved, and not block at arbitrary points.