185 lines
7.2 KiB
ReStructuredText
185 lines
7.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============
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NET_FAILOVER
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============
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Overview
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========
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The net_failover driver provides an automated failover mechanism via APIs
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to create and destroy a failover master netdev and manages a primary and
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standby slave netdevs that get registered via the generic failover
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infrastructure.
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The failover netdev acts a master device and controls 2 slave devices. The
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original paravirtual interface is registered as 'standby' slave netdev and
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a passthru/vf device with the same MAC gets registered as 'primary' slave
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netdev. Both 'standby' and 'failover' netdevs are associated with the same
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'pci' device. The user accesses the network interface via 'failover' netdev.
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The 'failover' netdev chooses 'primary' netdev as default for transmits when
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it is available with link up and running.
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This can be used by paravirtual drivers to enable an alternate low latency
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datapath. It also enables hypervisor controlled live migration of a VM with
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direct attached VF by failing over to the paravirtual datapath when the VF
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is unplugged.
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virtio-net accelerated datapath: STANDBY mode
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=============================================
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net_failover enables hypervisor controlled accelerated datapath to virtio-net
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enabled VMs in a transparent manner with no/minimal guest userspace changes.
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To support this, the hypervisor needs to enable VIRTIO_NET_F_STANDBY
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feature on the virtio-net interface and assign the same MAC address to both
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virtio-net and VF interfaces.
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Here is an example libvirt XML snippet that shows such configuration:
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::
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<interface type='network'>
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<mac address='52:54:00:00:12:53'/>
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<source network='enp66s0f0_br'/>
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<target dev='tap01'/>
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<model type='virtio'/>
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<driver name='vhost' queues='4'/>
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<link state='down'/>
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<teaming type='persistent'/>
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<alias name='ua-backup0'/>
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</interface>
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<interface type='hostdev' managed='yes'>
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<mac address='52:54:00:00:12:53'/>
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<source>
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<address type='pci' domain='0x0000' bus='0x42' slot='0x02' function='0x5'/>
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</source>
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<teaming type='transient' persistent='ua-backup0'/>
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</interface>
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In this configuration, the first device definition is for the virtio-net
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interface and this acts as the 'persistent' device indicating that this
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interface will always be plugged in. This is specified by the 'teaming' tag with
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required attribute type having value 'persistent'. The link state for the
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virtio-net device is set to 'down' to ensure that the 'failover' netdev prefers
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the VF passthrough device for normal communication. The virtio-net device will
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be brought UP during live migration to allow uninterrupted communication.
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The second device definition is for the VF passthrough interface. Here the
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'teaming' tag is provided with type 'transient' indicating that this device may
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periodically be unplugged. A second attribute - 'persistent' is provided and
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points to the alias name declared for the virtio-net device.
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Booting a VM with the above configuration will result in the following 3
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interfaces created in the VM:
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::
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4: ens10: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
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link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff
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inet 192.168.12.53/24 brd 192.168.12.255 scope global dynamic ens10
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valid_lft 42482sec preferred_lft 42482sec
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inet6 fe80::97d8:db2:8c10:b6d6/64 scope link
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valid_lft forever preferred_lft forever
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5: ens10nsby: <BROADCAST,MULTICAST> mtu 1500 qdisc fq_codel master ens10 state DOWN group default qlen 1000
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link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff
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7: ens11: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master ens10 state UP group default qlen 1000
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link/ether 52:54:00:00:12:53 brd ff:ff:ff:ff:ff:ff
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Here, ens10 is the 'failover' master interface, ens10nsby is the slave 'standby'
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virtio-net interface, and ens11 is the slave 'primary' VF passthrough interface.
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One point to note here is that some user space network configuration daemons
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like systemd-networkd, ifupdown, etc, do not understand the 'net_failover'
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device; and on the first boot, the VM might end up with both 'failover' device
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and VF acquiring IP addresses (either same or different) from the DHCP server.
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This will result in lack of connectivity to the VM. So some tweaks might be
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needed to these network configuration daemons to make sure that an IP is
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received only on the 'failover' device.
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Below is the patch snippet used with 'cloud-ifupdown-helper' script found on
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Debian cloud images:
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::
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@@ -27,6 +27,8 @@ do_setup() {
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local working="$cfgdir/.$INTERFACE"
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local final="$cfgdir/$INTERFACE"
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+ if [ -d "/sys/class/net/${INTERFACE}/master" ]; then exit 0; fi
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+
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if ifup --no-act "$INTERFACE" > /dev/null 2>&1; then
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# interface is already known to ifupdown, no need to generate cfg
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log "Skipping configuration generation for $INTERFACE"
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Live Migration of a VM with SR-IOV VF & virtio-net in STANDBY mode
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==================================================================
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net_failover also enables hypervisor controlled live migration to be supported
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with VMs that have direct attached SR-IOV VF devices by automatic failover to
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the paravirtual datapath when the VF is unplugged.
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Here is a sample script that shows the steps to initiate live migration from
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the source hypervisor. Note: It is assumed that the VM is connected to a
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software bridge 'br0' which has a single VF attached to it along with the vnet
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device to the VM. This is not the VF that was passthrough'd to the VM (seen in
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the vf.xml file).
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::
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# cat vf.xml
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<interface type='hostdev' managed='yes'>
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<mac address='52:54:00:00:12:53'/>
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<source>
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<address type='pci' domain='0x0000' bus='0x42' slot='0x02' function='0x5'/>
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</source>
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<teaming type='transient' persistent='ua-backup0'/>
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</interface>
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# Source Hypervisor migrate.sh
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#!/bin/bash
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DOMAIN=vm-01
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PF=ens6np0
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VF=ens6v1 # VF attached to the bridge.
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VF_NUM=1
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TAP_IF=vmtap01 # virtio-net interface in the VM.
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VF_XML=vf.xml
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MAC=52:54:00:00:12:53
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ZERO_MAC=00:00:00:00:00:00
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# Set the virtio-net interface up.
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virsh domif-setlink $DOMAIN $TAP_IF up
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# Remove the VF that was passthrough'd to the VM.
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virsh detach-device --live --config $DOMAIN $VF_XML
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ip link set $PF vf $VF_NUM mac $ZERO_MAC
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# Add FDB entry for traffic to continue going to the VM via
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# the VF -> br0 -> vnet interface path.
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bridge fdb add $MAC dev $VF
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bridge fdb add $MAC dev $TAP_IF master
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# Migrate the VM
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virsh migrate --live --persistent $DOMAIN qemu+ssh://$REMOTE_HOST/system
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# Clean up FDB entries after migration completes.
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bridge fdb del $MAC dev $VF
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bridge fdb del $MAC dev $TAP_IF master
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On the destination hypervisor, a shared bridge 'br0' is created before migration
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starts, and a VF from the destination PF is added to the bridge. Similarly an
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appropriate FDB entry is added.
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The following script is executed on the destination hypervisor once migration
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completes, and it reattaches the VF to the VM and brings down the virtio-net
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interface.
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::
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# reattach-vf.sh
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#!/bin/bash
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bridge fdb del 52:54:00:00:12:53 dev ens36v0
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bridge fdb del 52:54:00:00:12:53 dev vmtap01 master
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virsh attach-device --config --live vm01 vf.xml
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virsh domif-setlink vm01 vmtap01 down
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