[dpdk-dev] [PATCH v3 7/9] net/virtio: Add MTU feature support

[Maxime Coquelin]

On 04/05/2017 11:42 AM, Tan, Jianfeng wrote:
> Hi Maxime,
>
> Thank you for replying.
>
> On 4/5/2017 3:11 PM, Maxime Coquelin wrote:
>> Hi Jianfeng,
>>
>> On 04/05/2017 06:52 AM, Tan, Jianfeng wrote:
>>> Hi Maxime,
>>>
>>> Have some confusion about this feature. Please help confirm.
>>>
>>> (1) With this feature, we only support to advertise MTU value which is
>>> defined by QEMU to frontend and backend driver separately. (2) But it
>>> does not allow frontend driver to set a different MTU to QEMU and then
>>> to vhost backend.
>>>
>>> Correct?
>>> If it's correct, why not MTU works like (2)?
>>
>> Because idea is that the hosts advertises the maximum MTU value it
>> supports. The frontend driver is free to use a smaller value. The goal
>> of this change is to make possible to set a uniform MTU value across
>> the infrastructure, the management tools giving a hint to the guests on
>> the MTU value it should use.
>
> Based on that MTU is the maximum packet size that can be sent instead of
> that can be received:
> (1) Why vhost (as a device) does not drop any packets which are longer
> than MTU when dequeue()?
That's a good point.
As when MTU value is negotiated, the guest agrees not to send larger
packets. But we cannot trust the guest, so vhost needs to check the
packet length.

> (2) See some NICs also use MTU to calculate maximum packet size that can
> be received, like ixgbe, i40e, shall we also do that?
Can you give me some pointers to the code?

Thanks,
Maxime