On 2/19/2020 12:57 AM, Ye Xiaolong wrote: > 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> >
Applied to dpdk-next-net/master, thanks.
