From 06459cf9d0611354d9ecb2297cd4b6161e3bcac0 Mon Sep 17 00:00:00 2001 From: Ciara Loftus Date: Tue, 18 Feb 2020 14:03:59 +0000 Subject: [PATCH] doc: update MTU limitations in AF_XDP guide Explain how kernel driver RX buffer sizes affect the maximum MTU size in practice. Signed-off-by: Ciara Loftus Reviewed-by: Xiaolong Ye --- 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 b434b25df7..07bdd29e29 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. -- 2.20.1