1 .. SPDX-License-Identifier: GPL-2.0
2
3 ============
4 NET_FAILOVER
5 ============
6
7 Overview
8 ========
9
10 The net_failover driver provides an automated failover mechanism via APIs
11 to create and destroy a failover master netdev and manages a primary and
12 standby slave netdevs that get registered via the generic failover
13 infrastructure.
14
15 The failover netdev acts a master device and controls 2 slave devices. The
16 original paravirtual interface is registered as 'standby' slave netdev and
17 a passthru/vf device with the same MAC gets registered as 'primary' slave
18 netdev. Both 'standby' and 'failover' netdevs are associated with the same
19 'pci' device. The user accesses the network interface via 'failover' netdev.
20 The 'failover' netdev chooses 'primary' netdev as default for transmits when
21 it is available with link up and running.
22
23 This can be used by paravirtual drivers to enable an alternate low latency
24 datapath. It also enables hypervisor controlled live migration of a VM with
25 direct attached VF by failing over to the paravirtual datapath when the VF
26 is unplugged.
27
28 virtio-net accelerated datapath: STANDBY mode
29 =============================================
30
31 net_failover enables hypervisor controlled accelerated datapath to virtio-net
32 enabled VMs in a transparent manner with no/minimal guest userspace changes.
33
34 To support this, the hypervisor needs to enable VIRTIO_NET_F_STANDBY
35 feature on the virtio-net interface and assign the same MAC address to both
36 virtio-net and VF interfaces.
37
38 Here 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
59 In this configuration, the first device definition is for the virtio-net
60 interface and this acts as the 'persistent' device indicating that this
61 interface will always be plugged in. This is specified by the 'teaming' tag with
62 required attribute type having value 'persistent'. The link state for the
63 virtio-net device is set to 'down' to ensure that the 'failover' netdev prefers
64 the VF passthrough device for normal communication. The virtio-net device will
65 be brought UP during live migration to allow uninterrupted communication.
66
67 The second device definition is for the VF passthrough interface. Here the
68 'teaming' tag is provided with type 'transient' indicating that this device may
69 periodically be unplugged. A second attribute - 'persistent' is provided and
70 points to the alias name declared for the virtio-net device.
71
72 Booting a VM with the above configuration will result in the following 3
73 interfaces 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
87 Here, ens10 is the 'failover' master interface, ens10nsby is the slave 'standby'
88 virtio-net interface, and ens11 is the slave 'primary' VF passthrough interface.
89
90 One point to note here is that some user space network configuration daemons
91 like systemd-networkd, ifupdown, etc, do not understand the 'net_failover'
92 device; and on the first boot, the VM might end up with both 'failover' device
93 and VF acquiring IP addresses (either same or different) from the DHCP server.
94 This will result in lack of connectivity to the VM. So some tweaks might be
95 needed to these network configuration daemons to make sure that an IP is
96 received only on the 'failover' device.
97
98 Below is the patch snippet used with 'cloud-ifupdown-helper' script found on
99 Debian 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
113 Live Migration of a VM with SR-IOV VF & virtio-net in STANDBY mode
114 ==================================================================
115
116 net_failover also enables hypervisor controlled live migration to be supported
117 with VMs that have direct attached SR-IOV VF devices by automatic failover to
118 the paravirtual datapath when the VF is unplugged.
119
120 Here is a sample script that shows the steps to initiate live migration from
121 the source hypervisor. Note: It is assumed that the VM is connected to a
122 software bridge 'br0' which has a single VF attached to it along with the vnet
123 device to the VM. This is not the VF that was passthrough'd to the VM (seen in
124 the 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
169 On the destination hypervisor, a shared bridge 'br0' is created before migration
170 starts, and a VF from the destination PF is added to the bridge. Similarly an
171 appropriate FDB entry is added.
172
173 The following script is executed on the destination hypervisor once migration
174 completes, and it reattaches the VF to the VM and brings down the virtio-net
175 interface.
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
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