Re: [Qemu-devel] [QEMU] net: adapt dump to support the new syntax

[Stefan Hajnoczi]

On Tue, Jan 24, 2012 at 6:22 PM, Benjamin <address@hidden> wrote:
> Hello Stefan,

Hi,
Please send QEMU development emails to the mailing list so others can
help or we can avoid duplicating work in case someone else is already
looking into this.  I have CCed the mailing list.

> I've seen your blog and I hope you can help me address the issue of
> -net dump not being compatible with the new -netdev syntax.

The reason is because packet capture is implemented as a net client -
it receives packets on the "VLAN" and writes them to the pcap file.
In the -netdev model each client has a peer (another net client that
is communicates with).  So it's not possible to plug the "dump" net
client together with an emulated NIC client and a host tap client, for
example.  Since "VLANs" broadcast packets to all attached net clients
it works there.

> I think it is possible to achieve but I've had a lot of trouble
> understanding the code, VLANs are everywhere and even though the new
> syntax doesn't use them it looks like the code still uses a VLAN logic.
>
> Before I really start to get serious about this task, do you have any
> advice or recommendation?

I refactored the network subsystem to drop the "VLAN" concept a while
back but never got around to submitting the patches:

http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/vlan-hub

This branch completely removes the "VLAN" feature.  Instead it
introduces hubs, which are net clients that have multiple ports and
broadcast packets between them.  This allows you to achieve the same
behavior as "VLANs" except we remove all the hardcoded special cases
in the net subsystem and instead push that feature out into the hub
net client.

If you rebase the vlan-hub branch onto a recent qemu.git, then you can
use -netdev syntax to create an emulated NIC, host device, dump
device, and a hub.  The three clients must be attached to the hub.

I think the code is already there, it should work.  I didn't submit
this because I wanted to implement automated tests to ensure that
these changes don't brake the old syntax for "VLANs".

Feel free to play around with the vlan-hub branch, test it, and push
it upstream along with changes that you make.

Stefan