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

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

[Ciara Loftus]

Explain how kernel driver RX buffer sizes affect the maximum
MTU size in practice.

Signed-off-by: Ciara Loftus <ciara.loftus@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

Re: [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@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@intel.com>

Re: [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@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@intel.com>
> 

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