[libvirt] VLAN-based direct interface

In my setup, the network interfaces of several KVM-based VMs all communicate with the outside world via a single host interface, which is in turn connected to an external switch that handles all network policy to/from/between VMs. Traffic between the host and the switch is segregated by VLAN ID, one per vNIC. When I spin up a new VM with one vNIC assigned (say) VLAN ID 101, I run ip link add link eth0 name eth0.101 type vlan id 101. In the libvirt config I write <interface type='direct' trustGuestRxFilters='yes'> <source dev='eth0.101' mode='passthrough'/> <model type='virtio'/> </interface> This causes libvirt to create a macvtap interface attached to eth0.101 in passthrough mode. So far, so good. The guest OS thinks it has a plain old NIC on an untagged switch port. When the guest OS joins an IPv6 link-local solicited-node multicast group, QEMU tells libvirt to add the corresponding multicast MAC address to the macvtap interface. I can even run a trunk port all the way through to the VM, with traffic double-tagged through the switch-host hop. Trouble arises if I change the model from virtio to e1000, to support an application that doesn't (yet) have virtio drivers. QEMU doesn't expose state via query-rx-filter for e1000 interfaces, so libvirt can't sync multicast MAC addresses from the guest to the macvtap interface, so the guest can't talk IPv6. Ideally QEMU would support query-rx-filter for e1000 and other NIC models. But for my application all the careful syncing of guest rx filter state is overkill: I need the macvtap interface to pass all traffic between the guest and host with no filtering at all. I can achieve this simply by enabling promiscuous mode on the macvtap interface. But libvirt creates the interface, and I don't see any way to get libvirt to enable promiscuous mode and leave it enabled. It wouldn't be hard to add another configuration flag that tells libvirt to just put the macvtap interface into promiscuous mode and skip syncing guest rx filters. Perhaps add another value for trustGuestRxFilters, like 'promisc'? However just solving that minor problem seems like piling one hack on top of another: taking a normal interface and attaching a VLAN interface to it, then attaching a macvtap interface to that, and beating the mac out of the macvtap until it behaves like a dumb bridge (or, attaching a bridge to the VLAN interface, creating a tap interface, and enslaving the tap to the bridge, old-school). What I'm really after is just another kind of direct interface that attaches to a host interface, but instead of directing traffic to the guest by MAC address it uses the VLAN ID (and also adds/strips the tag). libvirt could implement this by automating all the steps I just described. Or if the kernel ever sprouts the VLAN equivalent of the macvtap driver (vlanvtap?), libvirt could just use that. Any opinions on first-class support for a VLAN-based direct interface, versus just allowing configuration of promiscuous mode on a macvtap direct interface? --Ed