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

[Daniel P. Berrange]

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()


Regards,
Daniel
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