QEMU virtio-net standby (net_failover)
This document explains the setup and usage of virtio-net standby feature which is used to create a net_failover pair of devices.
The general idea is that we have a pair of devices, a (vfio-)pci and a virtio-net device. Before migration the vfio device is unplugged and data flows through the virtio-net device, on the target side another vfio-pci device is plugged in to take over the data-path. In the guest the net_failover kernel module will pair net devices with the same MAC address.
The two devices are called primary and standby device. The fast hardware based networking device is called the primary device and the virtio-net device is the standby device.
Currently only PCIe devices are allowed as primary devices, this restriction can be lifted in the future with enhanced QEMU support. Also, only networking devices are allowed as primary device. The user needs to ensure that primary and standby devices are not plugged into the same PCIe slot.
Virtio-net standby allows easy migration while using a passed-through fast networking device by falling back to a virtio-net device for the duration of the migration. It is like a simple version of a bond, the difference is that it requires no configuration in the guest. When a guest is live-migrated to another host QEMU will unplug the primary device via the PCIe based hotplug handler and traffic will go through the virtio-net device. On the target system the primary device will be automatically plugged back and the net_failover module registers it again as the primary device.
The primary device can be hotplugged or be part of the startup configuration
- -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc,
With the parameter failover=on the VIRTIO_NET_F_STANDBY feature will be enabled.
failover_pair_id references the id of the virtio-net standby device. This is only for pairing the devices within QEMU. The guest kernel module net_failover will match devices with identical MAC addresses.
Both primary and standby device can be hotplugged via the QEMU monitor. Note that if the virtio-net device is plugged first a warning will be issued that it couldn’t find the primary device.
A new migration state wait-unplug was added for this feature. If failover primary devices are present in the configuration, migration will go into this state. It will wait until the device unplug is completed in the guest and then move into active state. On the target system the primary devices will be automatically hotplugged when the feature bit was negotiated for the virtio-net standby device.