This is the BPF program used to implement Receive Side Scaling (RSS)
across multiple queues if required by a flow action. The program is
loaded into the kernel when first RSS flow rule is created and is never unloaded.

When flow rules with the TAP device, packets are first handled by the
ingress queue discipline that then runs a series of classifier filter rules.
The first stage is the flow based classifier (flower); for RSS queue
action the second stage is an the kernel skbedit action which sets
the skb mark to a key based on the flow id; the final stage
is this BPF program which then maps flow id and packet header
into a queue id.

This version is built the BPF Compile Once — Run Everywhere (CO-RE)
framework and uses libbpf and bpftool.

Limitations
-----------
- requires libbpf to run

- rebuilding the BPF requires the clang compiler with bpf available
  as a target architecture and bpftool to convert object to headers.

  Some older versions of Ubuntu do not have a working bpftool package.

- only standard Toeplitz hash with standard 40 byte key is supported.

- the number of flow rules using RSS is limited to 32.

Building
--------
During the DPDK build process the meson build file checks that
libbpf, bpftool, and clang are available. If everything works then
BPF RSS is enabled.

The steps are:

1. Uses clang to compile tap_rss.c to produce tap_rss.bpf.o

2. Uses bpftool generate a skeleton header file tap_rss.skel.h
   from tap_rss.bpf.o. This header contains wrapper functions for
   managing the BPF and the actual BPF code as a large byte array.

3. The header file is include in tap_flow.c so that it can load
   the BPF code (via libbpf).

References
----------
BPF and XDP reference guide
https://docs.cilium.io/en/latest/bpf/progtypes/