Re: [Qemu-devel] [PATCH v3 00/16] net: hub-based networking

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Thu, May 24, 2012 at 05:53:21PM -0300, Luiz Capitulino wrote:
> On Fri, 25 May 2012 01:59:06 +0800
> zwu.kernel@gmail.com wrote:
> 
> > From: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
> > 
> > The patchset implements network hub stead of vlan. The main work was done by stefan, and i rebased it to latest QEMU upstream, did some testings and am responsible for pushing it to QEMU upstream.
> 
> Honest question: does it really pay off to have this in qemu vs. using one of
> the externaly available solutions?

For typical KVM setups this feature will be unused.

However, the legacy QEMU "vlan" feature does have a few uses:

1. It's how the "dump" netdev can be connected up with a guest NIC and
   host netdev.  Create a hub, attach the guest NIC, attach the host
   netdev, and attach the dump netdev.  This allows the dump netdev to
   see all traffic.

2. It lets you build virtual network segments.  Several point-to-point
   connections can be brought together.  Start 3 VMs connected by the
   "socket" netdev and have one of them use a hub.  This may be
   inefficient but I wouldn't be surprised if there are people out there
   doing this.

The point of this patch series is to remove the special-case net.c code
for the legacy "vlan" feature.  Today's code makes it harder to
implement a clean QOM model and is a burden for the net subsystem in
general.  This series takes the vlan functionality and puts it into a
normal netdev - it extracts the feature from net.c.

(If we didn't care about backwards compatibility we could just drop
vlans completely and rewrite the "dump" netdev to hook into the net.h
API somehow.)

Stefan