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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v3 00/16] net: hub-based networking
Date: Fri, 25 May 2012 13:01:37 +0100

On Fri, May 25, 2012 at 12:18 PM, Markus Armbruster <address@hidden> wrote:
> Stefan Hajnoczi <address@hidden> writes:
>> On Thu, May 24, 2012 at 05:53:21PM -0300, Luiz Capitulino wrote:
>>> On Fri, 25 May 2012 01:59:06 +0800
>>> address@hidden wrote:
>>> > From: Zhi Yong Wu <address@hidden>
>>> >
>>> > 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?

The aim is to keep the command-line vlan= syntax in tact.  The hub
change is mostly internal.

I agree it would be nice to drop entirely but I don't feel happy doing
that to users who might have QEMU buried in scripts somewhere.  One
day they upgrade packages and suddenly their stuff doesn't work


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