xsk: add FAQ to facilitate for first time users
Added an FAQ section in Documentation/networking/af_xdp.rst to help first time users with common problems. As problems are getting identified, entries will be added to the FAQ. Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
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@ -295,6 +295,41 @@ using::
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For XDP_SKB mode, use the switch "-S" instead of "-N" and all options
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can be displayed with "-h", as usual.
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FAQ
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=======
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Q: I am not seeing any traffic on the socket. What am I doing wrong?
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A: When a netdev of a physical NIC is initialized, Linux usually
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allocates one Rx and Tx queue pair per core. So on a 8 core system,
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queue ids 0 to 7 will be allocated, one per core. In the AF_XDP
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bind call or the xsk_socket__create libbpf function call, you
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specify a specific queue id to bind to and it is only the traffic
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towards that queue you are going to get on you socket. So in the
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example above, if you bind to queue 0, you are NOT going to get any
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traffic that is distributed to queues 1 through 7. If you are
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lucky, you will see the traffic, but usually it will end up on one
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of the queues you have not bound to.
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There are a number of ways to solve the problem of getting the
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traffic you want to the queue id you bound to. If you want to see
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all the traffic, you can force the netdev to only have 1 queue, queue
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id 0, and then bind to queue 0. You can use ethtool to do this::
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sudo ethtool -L <interface> combined 1
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If you want to only see part of the traffic, you can program the
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NIC through ethtool to filter out your traffic to a single queue id
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that you can bind your XDP socket to. Here is one example in which
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UDP traffic to and from port 4242 are sent to queue 2::
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sudo ethtool -N <interface> rx-flow-hash udp4 fn
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sudo ethtool -N <interface> flow-type udp4 src-port 4242 dst-port \
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4242 action 2
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A number of other ways are possible all up to the capabilitites of
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the NIC you have.
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Credits
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=======
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@ -309,4 +344,3 @@ Credits
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- Michael S. Tsirkin
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- Qi Z Zhang
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- Willem de Bruijn
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