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

[Ferruh Yigit]

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.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>
> 

Applied to dpdk-next-net/master, thanks.