[libvirt] [PATCH] qemu: change the name of tap device for a tap and bridge network

Daniel P. Berrange berrange at redhat.com
Tue May 2 08:05:35 UTC 2017

On Fri, Apr 28, 2017 at 01:08:45PM -0400, Laine Stump wrote:
> On 04/28/2017 05:33 AM, Daniel P. Berrange wrote:
> > On Fri, Apr 28, 2017 at 05:23:19PM +0800, ZhiPeng Lu wrote:
> >> Creating tap device and adding the device to bridge are not atomic operation.
> >> Similarly deleting tap device and removing it from bridge are not atomic operation.
> >> The Problem occurs when two vms start and shutdown. When one vm with the nic
> >> named "vnet0" stopping, it deleted tap device but not removing port from bridge.
> >> At this time, another vm created the tap device named "vnet0" and added port to the
> >> same bridge. Then, the first vm deleted the tap device from the same bridge.
> >> Finally, the tap device of the second vm don't attached to the bridge.
> >> So, we can add domid to vm's nic name. For example, the vm's domid is 1 and vnet0
> >> is renamed to vnet1.0.
> > 
> > Surely deleting the NIC automatically removes it from the bridge so we
> > can just remove the code that delets the bridge port.
> That is true when using a Linux host bridge, but I recall that for
> openvswitch (which I think is what ZhiPeng is using, based on an earlier
> patch), you must explicitly remove the port from the bridge - apparently
> the port is still there in openvswitch's table as some sort of "zombie"
> connection even after the tap device itself no longer exists.
> But instead of changing the naming scheme, maybe we should just delete
> the bridge port *before* deleting the tap device instead of after. (Am I
> recalling correctly that the tap device is deleted automatically when
> the qemu process is killed? If so, then what's needed is to move the
> loop in qemuProcessStop() that cleans up network interfaces so that it
> happens before qemuProcessKill() is called.

Agreed, we should just reverse the order.

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