[PATCH] net/af_xdp: update doc with information on queue setup

[Ciara Loftus <ciara.loftus@intel.com>]

When an AF_XDP PMD is created without specifying the 'start_queue', the
default Rx queue associated with the socket will be Rx queue 0. A common
scenario encountered by users new to AF_XDP is that they create the
socket on queue 0 however their interface is configured with many more
queues. In this case, traffic might land on for example queue 18 which
means it will never reach the socket.

This commit updates the AF_XDP documentation with instructions on how to
configure the interface to ensure the traffic will land on queue 0 and
thus reach the socket successfully.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
---
 doc/guides/nics/af_xdp.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/doc/guides/nics/af_xdp.rst b/doc/guides/nics/af_xdp.rst
index ee1e146a9e..ba3ee6a2f0 100644
--- a/doc/guides/nics/af_xdp.rst
+++ b/doc/guides/nics/af_xdp.rst
@@ -64,6 +64,16 @@ The following example will set up an af_xdp interface in DPDK:
 
     --vdev net_af_xdp,iface=ens786f1
 
+If 'start_queue' is not specified in the vdev arguments, the socket will by
+default be created on Rx queue 0. To ensure traffic lands on this queue, one can
+use flow steering if the network card supports it. Or, a simpler way is to
+reduce the number of configured queues for the device which will ensure that all
+traffic will land on queue 0 and thus reach the socket:
+
+.. code-block:: console
+
+    ethtool -L ens786f1 combined 1
+
 Limitations
 -----------
 
-- 
2.17.1