Current implementation only supports single queue, multi-queues feature will
be added later.
-Note that MTU of AF_XDP PMD is limited due to XDP lacks support for
-fragmentation.
-
AF_XDP PMD enables need_wakeup flag by default if it is supported. This
need_wakeup feature is used to support executing application and driver on the
same core efficiently. This feature not only has a large positive performance
* ``iface`` - name of the Kernel interface to attach to (required);
* ``start_queue`` - starting netdev queue id (optional, default 0);
* ``queue_count`` - total netdev queue number (optional, default 1);
-* ``pmd_zero_copy`` - enable zero copy or not (optional, default 0);
Prerequisites
-------------
<kernel src tree>/tools/lib/bpf;
* A Kernel bound interface to attach to;
* For need_wakeup feature, it requires kernel version later than v5.3-rc1;
+* For PMD zero copy, it requires kernel version later than v5.4-rc1;
Set up an af_xdp interface
-----------------------------
.. code-block:: console
--vdev net_af_xdp,iface=ens786f1
+
+Limitations
+-----------
+
+- **MTU**
+
+ The MTU of the AF_XDP PMD is limited due to the XDP requirement of one packet
+ per page. In the PMD we report the maximum MTU for zero copy to be equal
+ to the page size less the frame overhead introduced by AF_XDP (XDP HR = 256)
+ and DPDK (frame headroom = 320). With a 4K page size this works out at 3520.
+ However in practice this value may be even smaller, due to differences between
+ the supported RX buffer sizes of the underlying kernel netdev driver.
+
+ For example, the largest RX buffer size supported by the underlying kernel driver
+ which is less than the page size (4096B) may be 3072B. In this case, the maximum
+ MTU value will be at most 3072, but likely even smaller than this, once relevant
+ headers are accounted for eg. Ethernet and VLAN.
+
+ To determine the actual maximum MTU value of the interface you are using with the
+ AF_XDP PMD, consult the documentation for the kernel driver.
+
+ Note: The AF_XDP PMD will fail to initialise if an MTU which violates the driver's
+ conditions as above is set prior to launching the application.