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.lof...@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...@intel.com>
