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doc: update MTU limitations in AF_XDP guide

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Explain how kernel driver RX buffer sizes affect the maximum
MTU size in practice.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Reviewed-by: Xiaolong Ye <xiaolong.ye@intel.com>
This commit is contained in:
Ciara Loftus 2020-02-18 14:03:59 +00:00 committed by Ferruh Yigit
parent 366a0500f4
commit 06459cf9d0
1 changed files with 23 additions and 3 deletions
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26
doc/guides/nics/af_xdp.rst
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@ -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
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
@ -57,3 +54,26 @@ The following example will set up an af_xdp interface in DPDK:
.. 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.