RHEL6: QinQ (802.1ad) stacked VLAN tagging not working with SR-IOV VF devices

Issue

• A guest is configured internally to use one or more 802.1q vlan tags
• The host is configured (via libvirt) to place the guest's traffic on an 802.1q vlan under another tag

• The intention is that guest traffic appear on the physical network as QinQ / 802.1ad double tagged, with the inner tag supplied by the guest, and the outer tag supplied by the host.

• This works as expected when the guest uses virtio-net virtual NICs which are connected to the physical network via a bridge on the host.

• This does not work when the guest uses an SR-IOV Virtual Function of the host's NIC passed through to it.