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

[Michael S. Tsirkin]

On Tue, Dec 13, 2016 at 02:17:11PM +0100, Maxime Coquelin wrote:
> 
> 
> 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

I applied as is for now, we can rework as necessary but people want to
be able to test this.