RFC - Tap io_uring PMD

RFC - Tap io_uring PMD

[Stephen Hemminger]

The current tap device is slow both due to architectural choices and the
overhead of Linux system calls. I am exploring a how to fix that but some
of the choices require some tradeoffs. Which leads to some open questions:

1. DPDK tap also support tunnel (TUN) mode where there is no Ethernet header
   only L3. Does anyone actually use this? It is different than what every other
   PMD expects.

2. The fastest way to use kernel TAP device would be to use io_uring.
   But this was added in 5.1 kernel (2019). Rather than having conditional or
   dual mode in DPDK tap device, perhaps there should just be a new PMD tap_uring?

3. Current TAP device provides hooks for several rte_flow types by playing
   games with kernel qdisc. Does anyone really use this? Propose just not doing
   this in new tap_uring.

4. What other features of TAP device beyond basic send/receive make sense?
   It looks like new device could support better statistics.

5. What about Rx interrupt support?

Probably the hardest part of using io_uring is figuring out how to collect
completions. The simplest way would be to handle all completions rx and tx
in the rx_burst function.