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+.. SPDX-License-Identifier: GPL-2.0
+
+============
+NET_FAILOVER
+============
+
+Overview
+========
+
+The net_failover driver provides an automated failover mechanism via APIs
+to create and destroy a failover master netdev and manages a primary and
+standby slave netdevs that get registered via the generic failover
+infrastructure.
+
+The failover netdev acts a master device and controls 2 slave devices. The
+original paravirtual interface is registered as 'standby' slave netdev and
+a passthru/vf device with the same MAC gets registered as 'primary' slave
+netdev. Both 'standby' and 'failover' netdevs are associated with the same
+'pci' device. The user accesses the network interface via 'failover' netdev.
+The 'failover' netdev chooses 'primary' netdev as default for transmits when
+it is available with link up and running.
+
+This can be used by paravirtual drivers to enable an alternate low latency
+datapath. It also enables hypervisor controlled live migration of a VM with
+direct attached VF by failing over to the paravirtual datapath when the VF
+is unplugged.
+
+virtio-net accelerated datapath: STANDBY mode
+=============================================
+
+net_failover enables hypervisor controlled accelerated datapath to virtio-net
+enabled VMs in a transparent manner with no/minimal guest userspace changes.
+
+To support this, the hypervisor needs to enable VIRTIO_NET_F_STANDBY
+feature on the virtio-net interface and assign the same MAC address to both
+virtio-net and VF interfaces.
+
+Here is an example XML snippet that shows such configuration.
+::
+
+ <interface type='network'>
+ <mac address='52:54:00:00:12:53'/>
+ <source network='enp66s0f0_br'/>
+ <target dev='tap01'/>
+ <model type='virtio'/>
+ <driver name='vhost' queues='4'/>
+ <link state='down'/>
+ <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
+ </interface>
+ <interface type='hostdev' managed='yes'>
+ <mac address='52:54:00:00:12:53'/>
+ <source>
+ <address type='pci' domain='0x0000' bus='0x42' slot='0x02' function='0x5'/>
+ </source>
+ <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
+ </interface>
+
+Booting a VM with the above configuration will result in the following 3
+netdevs created in the VM.
+::
+
+ 4: ens10: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
+ link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff
+ inet 192.168.12.53/24 brd 192.168.12.255 scope global dynamic ens10
+ valid_lft 42482sec preferred_lft 42482sec
+ inet6 fe80::97d8:db2:8c10:b6d6/64 scope link
+ valid_lft forever preferred_lft forever
+ 5: ens10nsby: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel master ens10 state UP group default qlen 1000
+ link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff
+ 7: ens11: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master ens10 state UP group default qlen 1000
+ link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff
+
+ens10 is the 'failover' master netdev, ens10nsby and ens11 are the slave
+'standby' and 'primary' netdevs respectively.
+
+Live Migration of a VM with SR-IOV VF & virtio-net in STANDBY mode
+==================================================================
+
+net_failover also enables hypervisor controlled live migration to be supported
+with VMs that have direct attached SR-IOV VF devices by automatic failover to
+the paravirtual datapath when the VF is unplugged.
+
+Here is a sample script that shows the steps to initiate live migration on
+the source hypervisor.
+::
+
+ # cat vf_xml
+ <interface type='hostdev' managed='yes'>
+ <mac address='52:54:00:00:12:53'/>
+ <source>
+ <address type='pci' domain='0x0000' bus='0x42' slot='0x02' function='0x5'/>
+ </source>
+ <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
+ </interface>
+
+ # Source Hypervisor
+ #!/bin/bash
+
+ DOMAIN=fedora27-tap01
+ PF=enp66s0f0
+ VF_NUM=5
+ TAP_IF=tap01
+ VF_XML=
+
+ MAC=52:54:00:00:12:53
+ ZERO_MAC=00:00:00:00:00:00
+
+ virsh domif-setlink $DOMAIN $TAP_IF up
+ bridge fdb del $MAC dev $PF master
+ virsh detach-device $DOMAIN $VF_XML
+ ip link set $PF vf $VF_NUM mac $ZERO_MAC
+
+ virsh migrate --live $DOMAIN qemu+ssh://$REMOTE_HOST/system
+
+ # Destination Hypervisor
+ #!/bin/bash
+
+ virsh attach-device $DOMAIN $VF_XML
+ virsh domif-setlink $DOMAIN $TAP_IF down