[RFC {net,iproute2}-next 0/2] Introduce an eBPF hookpoint for tx queue selection in the XPS (Transmit Packet Steering) code.

[Matthew Cover]

WORK IN PROGRESS:
  * bpf program loading works!
  * txq steering via bpf program return code works!
  * bpf program unloading not working.
  * bpf program attached query not working.

This patch set provides a bpf hookpoint with goals similar to, but a more
generic implementation than, TUNSETSTEERINGEBPF; userspace supplied tx queue
selection policy.

TUNSETSTEERINGEBPF is a useful bpf hookpoint, but has some drawbacks.

First, it only works on tun/tap devices.

Second, there is no way in the current TUNSETSTEERINGEBPF implementation
to bail out or load a noop bpf prog and fallback to the no prog tx queue
selection method.

Third, the TUNSETSTEERINGEBPF interface seems to require possession of existing
or creation of new queues/fds.

This most naturally fits in the "wire" implementation since possession of fds
is ensured. However, it also means the various "wire" implementations (e.g.
qemu) have to all be made aware of TUNSETSTEERINGEBPF and expose an interface
to load/unload a bpf prog (or provide a mechanism to pass an fd to another
program).

Alternatively, you can spin up an extra queue and immediately disable via
IFF_DETACH_QUEUE, but this seems unsafe; packets could be enqueued to this
extra file descriptor which is part of our bpf prog loader, not our "wire".

Placing this in the XPS code and leveraging iproute2 and rtnetlink to provide
our bpf prog loader in a similar manner to xdp gives us a nice way to separate
the tap "wire" and the loading of tx queue selection policy. It also lets us
use this hookpoint for any device traversing XPS.

This patch only introduces the new hookpoint to the XPS code and will not yet
be used by tun/tap devices using the intree tun.ko (which implements an
.ndo_select_queue and does not traverse the XPS code).

In a future patch set, we can optionally refactor tun.ko to traverse this call
to bpf_prog_run_clear_cb() and bpf prog storage. tun/tap devices could then
leverage iproute2 as a generic loader. The TUNSETSTEERINGEBPF interface could
at this point be optionally deprecated/removed.

Both patches in this set have been tested using a rebuilt tun.ko with no
.ndo_select_queue.

  sed -i '/\.ndo_select_queue.*=/d' drivers/net/tun.c

The tap device was instantiated using tap_mq_pong.c, supporting scripts, and
wrapping service found here:

  https://github.com/stackpath/rxtxcpu/tree/v1.2.6/helpers

The bpf prog source and test scripts can be found here:

  https://github.com/werekraken/xps_ebpf

In nstxq, netsniff-ng using PACKET_FANOUT_QM is leveraged to check the
queue_mapping.

With no prog loaded, the tx queue selection is adhering our xps_cpus
configuration.

  [vagrant@localhost ~]$ grep . /sys/class/net/tap0/queues/tx-*/xps_cpus; ./nstxq; sudo timeout 1 cat /sys/kernel/debug/tracing/trace_pipe;
  /sys/class/net/tap0/queues/tx-0/xps_cpus:1
  /sys/class/net/tap0/queues/tx-1/xps_cpus:2
  cpu0: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.146 ms
  cpu0: qm0:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
  cpu1: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.121 ms
  cpu1: qm1:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0

With a return 0 bpg prog, our tx queue is 0 (despite xps_cpus).

  [vagrant@localhost ~]$ sudo ip link set dev tap0 xps obj hello0.o sec hello && { ./nstxq; sudo timeout 1 cat /sys/kernel/debug/tracing/trace_pipe; }
  cpu0: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.160 ms
  cpu0: qm0:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
  cpu1: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.124 ms
  cpu1: qm0:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
              ping-4852  [000] ....  2691.633260: 0: xps (RET 0): Hello, World!
              ping-4869  [001] ....  2695.753588: 0: xps (RET 0): Hello, World!

With a return 1 bpg prog, our tx queue is 1.

  [vagrant@localhost ~]$ sudo ip link set dev tap0 xps obj hello1.o sec hello && { ./nstxq; sudo timeout 1 cat /sys/kernel/debug/tracing/trace_pipe; }
  cpu0: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.193 ms
  cpu0: qm1:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
  cpu1: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.135 ms
  cpu1: qm1:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
              ping-4894  [000] ....  2710.652080: 0: xps (RET 1): Hello, World!
              ping-4911  [001] ....  2714.774608: 0: xps (RET 1): Hello, World!

With a return 2 bpg prog, our tx queue is 0 (we only have 2 tx queues).

  [vagrant@localhost ~]$ sudo ip link set dev tap0 xps obj hello2.o sec hello && { ./nstxq; sudo timeout 1 cat /sys/kernel/debug/tracing/trace_pipe; }
  cpu0: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=1.20 ms
  cpu0: qm0:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
  cpu1: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.986 ms
  cpu1: qm0:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
              ping-4936  [000] ....  2729.442668: 0: xps (RET 2): Hello, World!
              ping-4953  [001] ....  2733.614558: 0: xps (RET 2): Hello, World!

With a return -1 bpf prog, our tx queue selection is once again determined by
xps_cpus. Any negative return should work the same and provides a nice
mechanism to bail out or have a noop bpf prog at this hookpoint.

  [vagrant@localhost ~]$ sudo ip link set dev tap0 xps obj hello_neg1.o sec hello && { ./nstxq; sudo timeout 1 cat /sys/kernel/debug/tracing/trace_pipe; }
  cpu0: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.628 ms
  cpu0: qm0:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
  cpu1: ping: 64 bytes from 169.254.254.1: icmp_seq=1 ttl=64 time=0.322 ms
  cpu1: qm1:  > tap0 98 Unknown => Unknown IPv4 169.254.254.2/169.254.254.1 Len 84 Type 8 Code 0
              ping-4981  [000] ....  2763.510760: 0: xps (RET -1): Hello, World!
              ping-4998  [001] ....  2767.632583: 0: xps (RET -1): Hello, World!

bpf prog unloading is not yet working and neither does `ip link show` report
when an "xps" bpf prog is attached. This is my first time touching iproute2 or
rtnetlink, so it may be something obvious to those more familiar.