Re: [libvirt] high outage times for qemu virtio network links during live migration, trying to debug

I'm using libvirt (1.2.12) with qemu (2.2.0) in the context of OpenStack. If I live-migrate a guest with virtio network interfaces, I see a ~1200msec delay in processing the network packets, and several hundred of them get dropped. I get the dropped packets, but I'm not sure why the delay is there. I instrumented qemu and libvirt, and the strange thing is that this delay seems to happen before qemu actually starts doing any migration-related work. (i.e. before qmp_migrate() is called) Looking at my timestamps, the start of the glitch seems to coincide with libvirtd calling qemuDomainMigratePrepareTunnel3Params(), and the end of the glitch occurs when the migration is complete and we're up and running on the destination. My question is, why doesn't qemu continue processing virtio packets while the dirty page scanning and memory transfer over the network is proceeding?