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

From: Mike Lovell
Subject: Re: [Qemu-discuss] qemu guests using 802.1q vlans bridged on host
Date: Thu, 22 Aug 2013 14:03:05 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7

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.

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.


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