[dpdk-dev] [PATCH] doc: update af_xdp documentation on MTU limitations

[Ye Xiaolong]

Nice doc about MTU of AF_XDP, thanks for the update.

On 02/18, Ciara Loftus wrote:
>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
>--- a/doc/guides/nics/af_xdp.rst
>+++ b/doc/guides/nics/af_xdp.rst
>@@ -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.
>-- 
>2.25.0
>

Reviewed-by: Xiaolong Ye <xiaolong.ye at intel.com>