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

[Tan, Jianfeng]

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()?
(2) See some NICs also use MTU to calculate maximum packet size that can 
be received, like ixgbe, i40e, shall we also do that?

>
> Having the frontend setting the backend MTU dynamically would require 
> some negotiation at runtime, and is out of the scope of this change.
> Do you have some use cases for this?

No, just to understand more about this feature. Thank you!

Thanks,
Jianfeng

>
> Regards,
> Maxime
>> Thanks,
>> Jianfeng