1====================================== 2QEMU virtio-net standby (net_failover) 3====================================== 4 5This document explains the setup and usage of virtio-net standby feature which 6is used to create a net_failover pair of devices. 7 8The general idea is that we have a pair of devices, a (vfio-)pci and a 9virtio-net device. Before migration the vfio device is unplugged and data flows 10through the virtio-net device, on the target side another vfio-pci device is 11plugged in to take over the data-path. In the guest the net_failover kernel 12module will pair net devices with the same MAC address. 13 14The two devices are called primary and standby device. The fast hardware based 15networking device is called the primary device and the virtio-net device is the 16standby device. 17 18Restrictions 19------------ 20 21Currently only PCIe devices are allowed as primary devices, this restriction 22can be lifted in the future with enhanced QEMU support. Also, only networking 23devices are allowed as primary device. The user needs to ensure that primary 24and standby devices are not plugged into the same PCIe slot. 25 26Usecase 27------- 28 29 Virtio-net standby allows easy migration while using a passed-through fast 30 networking device by falling back to a virtio-net device for the duration of 31 the migration. It is like a simple version of a bond, the difference is that it 32 requires no configuration in the guest. When a guest is live-migrated to 33 another host QEMU will unplug the primary device via the PCIe based hotplug 34 handler and traffic will go through the virtio-net device. On the target 35 system the primary device will be automatically plugged back and the 36 net_failover module registers it again as the primary device. 37 38Usage 39----- 40 41 The primary device can be hotplugged or be part of the startup configuration 42 43 -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc, \ 44 bus=root2,failover=on 45 46 With the parameter failover=on the VIRTIO_NET_F_STANDBY feature will be enabled. 47 48 -device vfio-pci,host=5e:00.2,id=hostdev0,bus=root1,failover_pair_id=net1 49 50 failover_pair_id references the id of the virtio-net standby device. This 51 is only for pairing the devices within QEMU. The guest kernel module 52 net_failover will match devices with identical MAC addresses. 53 54Hotplug 55------- 56 57 Both primary and standby device can be hotplugged via the QEMU monitor. Note 58 that if the virtio-net device is plugged first a warning will be issued that it 59 couldn't find the primary device. 60 61Migration 62--------- 63 64 A new migration state wait-unplug was added for this feature. If failover primary 65 devices are present in the configuration, migration will go into this state. 66 It will wait until the device unplug is completed in the guest and then move into 67 active state. On the target system the primary devices will be automatically hotplugged 68 when the feature bit was negotiated for the virtio-net standby device. 69