1.. SPDX-License-Identifier: GPL-2.0 2 3============ 4NET_FAILOVER 5============ 6 7Overview 8======== 9 10The net_failover driver provides an automated failover mechanism via APIs 11to create and destroy a failover master netdev and manages a primary and 12standby slave netdevs that get registered via the generic failover 13infrastructure. 14 15The failover netdev acts a master device and controls 2 slave devices. The 16original paravirtual interface is registered as 'standby' slave netdev and 17a passthru/vf device with the same MAC gets registered as 'primary' slave 18netdev. Both 'standby' and 'failover' netdevs are associated with the same 19'pci' device. The user accesses the network interface via 'failover' netdev. 20The 'failover' netdev chooses 'primary' netdev as default for transmits when 21it is available with link up and running. 22 23This can be used by paravirtual drivers to enable an alternate low latency 24datapath. It also enables hypervisor controlled live migration of a VM with 25direct attached VF by failing over to the paravirtual datapath when the VF 26is unplugged. 27 28virtio-net accelerated datapath: STANDBY mode 29============================================= 30 31net_failover enables hypervisor controlled accelerated datapath to virtio-net 32enabled VMs in a transparent manner with no/minimal guest userspace changes. 33 34To support this, the hypervisor needs to enable VIRTIO_NET_F_STANDBY 35feature on the virtio-net interface and assign the same MAC address to both 36virtio-net and VF interfaces. 37 38Here is an example libvirt XML snippet that shows such configuration: 39:: 40 41 <interface type='network'> 42 <mac address='52:54:00:00:12:53'/> 43 <source network='enp66s0f0_br'/> 44 <target dev='tap01'/> 45 <model type='virtio'/> 46 <driver name='vhost' queues='4'/> 47 <link state='down'/> 48 <teaming type='persistent'/> 49 <alias name='ua-backup0'/> 50 </interface> 51 <interface type='hostdev' managed='yes'> 52 <mac address='52:54:00:00:12:53'/> 53 <source> 54 <address type='pci' domain='0x0000' bus='0x42' slot='0x02' function='0x5'/> 55 </source> 56 <teaming type='transient' persistent='ua-backup0'/> 57 </interface> 58 59In this configuration, the first device definition is for the virtio-net 60interface and this acts as the 'persistent' device indicating that this 61interface will always be plugged in. This is specified by the 'teaming' tag with 62required attribute type having value 'persistent'. The link state for the 63virtio-net device is set to 'down' to ensure that the 'failover' netdev prefers 64the VF passthrough device for normal communication. The virtio-net device will 65be brought UP during live migration to allow uninterrupted communication. 66 67The second device definition is for the VF passthrough interface. Here the 68'teaming' tag is provided with type 'transient' indicating that this device may 69periodically be unplugged. A second attribute - 'persistent' is provided and 70points to the alias name declared for the virtio-net device. 71 72Booting a VM with the above configuration will result in the following 3 73interfaces created in the VM: 74:: 75 76 4: ens10: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000 77 link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff 78 inet 192.168.12.53/24 brd 192.168.12.255 scope global dynamic ens10 79 valid_lft 42482sec preferred_lft 42482sec 80 inet6 fe80::97d8:db2:8c10:b6d6/64 scope link 81 valid_lft forever preferred_lft forever 82 5: ens10nsby: <BROADCAST,MULTICAST> mtu 1500 qdisc fq_codel master ens10 state DOWN group default qlen 1000 83 link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff 84 7: ens11: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master ens10 state UP group default qlen 1000 85 link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff 86 87Here, ens10 is the 'failover' master interface, ens10nsby is the slave 'standby' 88virtio-net interface, and ens11 is the slave 'primary' VF passthrough interface. 89 90One point to note here is that some user space network configuration daemons 91like systemd-networkd, ifupdown, etc, do not understand the 'net_failover' 92device; and on the first boot, the VM might end up with both 'failover' device 93and VF acquiring IP addresses (either same or different) from the DHCP server. 94This will result in lack of connectivity to the VM. So some tweaks might be 95needed to these network configuration daemons to make sure that an IP is 96received only on the 'failover' device. 97 98Below is the patch snippet used with 'cloud-ifupdown-helper' script found on 99Debian cloud images: 100 101:: 102 @@ -27,6 +27,8 @@ do_setup() { 103 local working="$cfgdir/.$INTERFACE" 104 local final="$cfgdir/$INTERFACE" 105 106 + if [ -d "/sys/class/net/${INTERFACE}/master" ]; then exit 0; fi 107 + 108 if ifup --no-act "$INTERFACE" > /dev/null 2>&1; then 109 # interface is already known to ifupdown, no need to generate cfg 110 log "Skipping configuration generation for $INTERFACE" 111 112 113Live Migration of a VM with SR-IOV VF & virtio-net in STANDBY mode 114================================================================== 115 116net_failover also enables hypervisor controlled live migration to be supported 117with VMs that have direct attached SR-IOV VF devices by automatic failover to 118the paravirtual datapath when the VF is unplugged. 119 120Here is a sample script that shows the steps to initiate live migration from 121the source hypervisor. Note: It is assumed that the VM is connected to a 122software bridge 'br0' which has a single VF attached to it along with the vnet 123device to the VM. This is not the VF that was passthrough'd to the VM (seen in 124the vf.xml file). 125:: 126 127 # cat vf.xml 128 <interface type='hostdev' managed='yes'> 129 <mac address='52:54:00:00:12:53'/> 130 <source> 131 <address type='pci' domain='0x0000' bus='0x42' slot='0x02' function='0x5'/> 132 </source> 133 <teaming type='transient' persistent='ua-backup0'/> 134 </interface> 135 136 # Source Hypervisor migrate.sh 137 #!/bin/bash 138 139 DOMAIN=vm-01 140 PF=ens6np0 141 VF=ens6v1 # VF attached to the bridge. 142 VF_NUM=1 143 TAP_IF=vmtap01 # virtio-net interface in the VM. 144 VF_XML=vf.xml 145 146 MAC=52:54:00:00:12:53 147 ZERO_MAC=00:00:00:00:00:00 148 149 # Set the virtio-net interface up. 150 virsh domif-setlink $DOMAIN $TAP_IF up 151 152 # Remove the VF that was passthrough'd to the VM. 153 virsh detach-device --live --config $DOMAIN $VF_XML 154 155 ip link set $PF vf $VF_NUM mac $ZERO_MAC 156 157 # Add FDB entry for traffic to continue going to the VM via 158 # the VF -> br0 -> vnet interface path. 159 bridge fdb add $MAC dev $VF 160 bridge fdb add $MAC dev $TAP_IF master 161 162 # Migrate the VM 163 virsh migrate --live --persistent $DOMAIN qemu+ssh://$REMOTE_HOST/system 164 165 # Clean up FDB entries after migration completes. 166 bridge fdb del $MAC dev $VF 167 bridge fdb del $MAC dev $TAP_IF master 168 169On the destination hypervisor, a shared bridge 'br0' is created before migration 170starts, and a VF from the destination PF is added to the bridge. Similarly an 171appropriate FDB entry is added. 172 173The following script is executed on the destination hypervisor once migration 174completes, and it reattaches the VF to the VM and brings down the virtio-net 175interface. 176 177:: 178 # reattach-vf.sh 179 #!/bin/bash 180 181 bridge fdb del 52:54:00:00:12:53 dev ens36v0 182 bridge fdb del 52:54:00:00:12:53 dev vmtap01 master 183 virsh attach-device --config --live vm01 vf.xml 184 virsh domif-setlink vm01 vmtap01 down 185