[dpdk-dev] [PATCH] doc: update af_xdp documentation on MTU limitations
ciara.loftus at intel.com
Tue Feb 18 15:03:59 CET 2020
Explain how kernel driver RX buffer sizes affect the maximum
MTU size in practice.
Signed-off-by: Ciara Loftus <ciara.loftus at intel.com>
doc/guides/nics/af_xdp.rst | 26 +++++++++++++++++++++++---
1 file changed, 23 insertions(+), 3 deletions(-)
diff --git a/doc/guides/nics/af_xdp.rst b/doc/guides/nics/af_xdp.rst
index b434b25df..07bdd29e2 100644
@@ -18,9 +18,6 @@ packets through the socket which would bypass the kernel network stack.
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
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
@@ -57,3 +54,26 @@ The following example will set up an af_xdp interface in DPDK:
.. code-block:: console
+ 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.
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