xsk: add FAQ to facilitate for first time users

For XDP_SKB mode, use the switch "-S" instead of "-N" and all options

can be displayed with "-h", as usual.

FAQ

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Q: I am not seeing any traffic on the socket. What am I doing wrong?

A: When a netdev of a physical NIC is initialized, Linux usually

   allocates one Rx and Tx queue pair per core. So on a 8 core system,

   queue ids 0 to 7 will be allocated, one per core. In the AF_XDP

   bind call or the xsk_socket__create libbpf function call, you

   specify a specific queue id to bind to and it is only the traffic

   towards that queue you are going to get on you socket. So in the

   example above, if you bind to queue 0, you are NOT going to get any

   traffic that is distributed to queues 1 through 7. If you are

   lucky, you will see the traffic, but usually it will end up on one

   of the queues you have not bound to.

   There are a number of ways to solve the problem of getting the

   traffic you want to the queue id you bound to. If you want to see

   all the traffic, you can force the netdev to only have 1 queue, queue

   id 0, and then bind to queue 0. You can use ethtool to do this::

   sudo ethtool -L <interface> combined 1

   If you want to only see part of the traffic, you can program the

   NIC through ethtool to filter out your traffic to a single queue id

   that you can bind your XDP socket to. Here is one example in which

   UDP traffic to and from port 4242 are sent to queue 2::

   sudo ethtool -N <interface> rx-flow-hash udp4 fn

   sudo ethtool -N <interface> flow-type udp4 src-port 4242 dst-port \

   4242 action 2

   A number of other ways are possible all up to the capabilitites of

   the NIC you have.

Credits

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- Michael S. Tsirkin

- Qi Z Zhang

- Willem de Bruijn