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2QEMU virtio-net standby (net_failover)
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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