[libvirt] RFC decoupling VM NIC provisioning from VM NIC connection to backend networks

Hi, In its current implementation Libvirt makes sure that the network interfaces that it passes/provision to a VM (for example to qemu[-kvm]) are already connected to its backend (interfaces/networks) by the time the VM starts its boot process. In a non virtualized setup it would be like booting a machine with the Ethernet cable already plugged into a router/switch port. While in a non virtualized setup you can boot a machine first (with no physical connection to a router/switch) and later connect its NIC/s to the switch/router, when you boot a VM via Libvirt it is not possible to decouple the two actions (VM boot, cable plug/unplug). An example of case where the capability of decoupling the two actions mentioned above is a requirement in Quantum/NetStack which is the network service leveraged by OpenStack. The modular design of OpenStack allows you to: - provision VMs with NIC/s - create networks - create ports on networks - plug/unplug a VM NIC into/from a given port on a network (at runtime) Note that this runtime plug/unplug requirement has nothing to do with hot plug/unplug of NICs. The idea is more that of decoupling the provisioning of a VM from the connection of the VM to the network/s. This would make it possible to change (at run-time too) the networks the NIC/s of a given VM are connected to. For example, when a VM boots, its interfaces should be in link down state if the network admin has not connected the VM NIC/s to any "network" yet. Even though libvirt already provides a way to change the link state of an a VM NIC, link state and physical connection are two different things and should be manageable independently. Ideally the configuration syntax should be interface type and hypervisor type agnostic. Let's take QEMU[-kvm] as an example - when Libvirt starts a QEMU VM, it passes to QEMU a number of file descriptors that map to host backend interfaces (for example macvtap interfaces). In order to introduce this runtime plug/unplug capability, we need a mechanism that permits to delay the binding between the host macvtap interfaces and the guest taps (because you cannot know the fd of the macvtap interfaces before you create them). This means you need a mechanism that allows you to change such fd/s at runtime: - you can close/reset an fd (ie, when you disconnect a VM NIC from its network) - you can open/set an fd (ie, when you connect a VM NIC to a network) This could probably be a libvirt command that translates to a QEMU monitor command. Can the runtime plug/unplug capability described above be achieved (cleanly) with another mechanism? Is anybody working on implementing something similar? [For more information on OpenStack/NetStack/Quantum and the above requirements please refer to the network model used therein: http://docs.openstack.org/incubation/openstack-network/admin/content/Wha tIsQuantum.html (information on network, port, and attachment abstractions) http://www.slideshare.net/danwent/quantum-diablo-summary (slides 7 & 8)] Thanks, ~Sumit Naiksatam. (On behalf of OpenStack/Quantum team)