Re: [Qemu-devel] [PATCH v4 0/3] virtio-net: Add support to MTU feature

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

On 12/12/2016 11:34 AM, Daniel P. Berrange wrote:
> On Mon, Dec 12, 2016 at 11:12:56AM +0100, Maxime Coquelin wrote:
>> Hi Daniel,
>>
>> On 12/12/2016 11:02 AM, Daniel P. Berrange wrote:
>>> On Sat, Dec 10, 2016 at 04:30:35PM +0100, Maxime Coquelin wrote:
>>>> Thanks for the reviews,
>>>>
>>>> This series implements Virtio spec update from Aaron Conole which
>>>> defines a way for the host to expose its max MTU to the guest.
>>>>
>>>> "host_mtu" parameter is added to provide QEMU with the MTU value,
>>>> and the backend, if supported, gets notified of the MTU value when the
>>>> MTU feature neogotiation succeeds.
>>>>
>>>> Only user backend currently supports MTU notification. A new protocol
>>>> feature has been implemented for sending MTU value to the backend.
>>>>
>>>> For kernel backend, it is expected the management tool also configures
>>>> the tap/macvtap interface with same MTU value.
>>>> Daniel, I would be interrested about your feedback on this implementation
>>>> from management tool point of view.
>>>
>>> I can't give real feedback yet, as I'm not seeing clear information on
>>> what problem this series is designed to solve....
>>
>> Right, I agree it is missing a bit of context here, I'll add more about
>> the background in next revision.
>>
>> The goal of this series is to address two things:
>> 1. Providing a way for the guests to use the same MTU as the host,
>>    in order to have a consistent MTU value across the infrastructure.
>
> Ok, currently libvirt sets the MTU of the tap device based on the MTU
> of the device it will be attached to. This change means we need to pass
> that MTU value into QEMU via the -netdev command line so it can inform
> the guest what the MTU is.
Nice, do you have a pointer on where this is done in QEMU?

Thanks,
Maxime