Re: [Qemu-discuss] qemu guests using 802.1q vlans bridged on host

[Mike Lovell]

On 08/22/2013 12:22 PM, Stephan von Krawczynski wrote:
> On Thu, 22 Aug 2013 11:55:28 -0600
> Mike Lovell <address@hidden> wrote:
>
> > On 08/22/2013 08:39 AM, Stephan von Krawczynski wrote:
> > > Hello all,
> > >
> > > I'd like to do something very simple - at least that's what I thought ;-)
> > > I want a guest to have access to a network just as if he was connected to the
> > > real card, but set up as bridge on the host and virtio network driver. The
> > > guest should be able to configure and use some or maybe even many 802.1q vlans
> > > on this network and the traffic should go out tagged.
> > >
> > > So I setup the hosts bridge and connected an intel network card and a qemu
> > > virtio card. Now the problem: No vlan-tagged traffic from the physical
> > > interface reaches the guest at all, and no vlan-tagged traffic from the guest
> > > reaches the physical net over the bridge. One major reason for this is the
> > > vlan offloading by the host interface card (intel). Another seems to be that
> > > arp requests are somehow not going through the bridge for the vlans.
> > >
> > > I hope that someone here has used 802.1q vlans inside guests before and can
> > > share some tips how to make this work. Because out-of-the-box it does not. All
> > > system are linux of course and with latest kernels (3.10.9 currently).
> > > qemu is 1.5.2.
> > > Thanks for any hints.
> > i'm guessing that you are using the standard linux bridge and i believe
> > that would be the problem. i don't think it supports vlan tags at all.
> > you should try using openvswitch as the virtual switch connecting the
> > physical and guest interfaces. it supports vlans and tagging and i think
> > you will be able to get tagged vlans all the way to the guest interfaces
> > by using it.
> >
> > mike
> I don't trust openvswitch regarding performance and latency. AFAIK linux
> bridge combined with vhost_net is zero-copy, right?
> What about openvswitch?

i unfortunately cannot answer those. i've only done enough with 
openvswitch to do a simple installation to try it. i've never done 
performance or latency testing myself. a quick search resulted in 
http://networkheresy.com/2012/06/08/the-overhead-of-software-tunneling/ 
which has a throughput comparison that includes the linux bridge and 
openvswitch. i believe you would be able to still use vhost_net with 
openvswitch since vhost_net is related to the interaction of the tap 
device with the guest processes and not between the tap device and the 
switch.

mike