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>
This commit is contained in:
Magnus Karlsson 2019-02-21 10:21:28 +01:00 committed by Daniel Borkmann
parent 248c7f9c0e
commit 0f4a9b7d4e
1 changed files with 35 additions and 1 deletions

View File

@ -295,6 +295,41 @@ using::
For XDP_SKB mode, use the switch "-S" instead of "-N" and all options
can be displayed with "-h", as usual.
FAQ
=======
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
=======
@ -309,4 +344,3 @@ Credits
- Michael S. Tsirkin
- Qi Z Zhang
- Willem de Bruijn