On Fri, May 25, 2012 at 12:18 PM, Markus Armbruster <armbru@xxxxxxxxxx> wrote: > Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx wrote: >>> >>> > From: Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx> >>> > >>> > 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 anymore. Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
