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

[Markus Armbruster <armbru@redhat.com>]

Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> writes:

> 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.

Those people will find other, superior ways to do this once this
inefficient way is gone.  We'd do them a favour, I'd say ;)

> 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.

Welcome improvement, but...

> (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.)

... is backward compatiblity really worth the extra net client?

Please excuse my ignorant question (I haven't studied the series): what
kind of backward compatiblity exactly do we get here?  Command line:
-net option vlan still works?  Or just semantic: whatever you could do
with vlans you can now do with hubs?

In case it's the latter: well, whatever you could do with vlans you can
already do with external software, can't you?  Why is it worthwhile to
provide yet another way within QEMU?