Re: [PATCH v3 4/9] vhost: Add API to get MTU value

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 03/17/2017 06:32 AM, Yuanhan Liu wrote:
> On Thu, Mar 16, 2017 at 12:37:23PM +0100, Maxime Coquelin wrote:
>>
>>
>> On 03/16/2017 09:00 AM, Yuanhan Liu wrote:
>>> On Sun, Mar 12, 2017 at 05:34:01PM +0100, Maxime Coquelin wrote:
>>>> This patch implements the function for the application to
>>>> get the MTU value.
>>>
>>> I'm thinking the other way. As we are making vhost being generic, it
>>> doesn't make too much sense now to store a net specific value at vhost
>>> layer. I'm thinking we may could introduce a vhost-user request handler,
>>> something like:
>>>
>>> 	rte_vhost_driver_register_msg_handler(socket, handler);
>>
>> That's a good point.
>>
>>> All vhost-user message then will goto the driver's sepcific handler.
>>> if it's handlered, do nothing in vhost lib. Otherwise, we handle it
>>> in vhost lib.
>>>
>>> In this way, you could handle the mtu message inside vhost-pmd; thus,
>>> there is no need to introduce one more (net specific) API.
>>>
>>> Thoughts?
>>
>> I need to think more about it, but advantage of having a dedicated API
>> is that in case the MTU value is not available, you can know from
>> return code whether it is not yet available (-EAGAIN), or not supported
>> (-ENOTSUP).
>>
>> That could be managed with the callback tough, by calling the callback
>> with a 0 mtu value if not supported, so that the application can be
>> sure that if the callback hasn't been called, then it is just that it
>> is not ready yet.
>>
>> What do you think?
>
> I don't think the application should even be aware of the callback.
> Application should get the mtu from the ethdev layer, by the API
> rte_eth_dev_get_mtu(). And such MTU request should be only handled
> in vhost-pmd, to serve the rte_eth_dev_get_mtu() API.

I thought about OVS, which doesn't rely on Vhost PMD (at least didn't
last time I checked).

Maxime