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

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

On 12/13/2016 02:07 PM, Daniel P. Berrange wrote:
> On Tue, Dec 13, 2016 at 02:04:52PM +0100, Maxime Coquelin wrote:
>>
>>
>> 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?
>
> Its in libvirt code via virNetDevSetMTUFromDevice()

Thanks.
For QEMU part, this series adds host_mtu parameter to virtio-net device,
not to vhost netdev. Should it be reworked?

Maxime