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<section id="qemu-virtio-net-standby-net-failover">
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<h1>QEMU virtio-net standby (net_failover)<a class="headerlink" href="#qemu-virtio-net-standby-net-failover" title="Link to this heading"></a></h1>
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<p>This document explains the setup and usage of virtio-net standby feature which
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is used to create a net_failover pair of devices.</p>
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<p>The general idea is that we have a pair of devices, a (vfio-)pci and a
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virtio-net device. Before migration the vfio device is unplugged and data flows
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through the virtio-net device, on the target side another vfio-pci device is
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plugged in to take over the data-path. In the guest the net_failover kernel
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module will pair net devices with the same MAC address.</p>
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<p>The two devices are called primary and standby device. The fast hardware based
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networking device is called the primary device and the virtio-net device is the
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standby device.</p>
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<section id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Link to this heading"></a></h2>
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<p>Currently only PCIe devices are allowed as primary devices, this restriction
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can be lifted in the future with enhanced QEMU support. Also, only networking
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devices are allowed as primary device. The user needs to ensure that primary
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and standby devices are not plugged into the same PCIe slot.</p>
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</section>
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<section id="usecase">
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<h2>Usecase<a class="headerlink" href="#usecase" title="Link to this heading"></a></h2>
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<blockquote>
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<div><p>Virtio-net standby allows easy migration while using a passed-through fast
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networking device by falling back to a virtio-net device for the duration of
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the migration. It is like a simple version of a bond, the difference is that it
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requires no configuration in the guest. When a guest is live-migrated to
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another host QEMU will unplug the primary device via the PCIe based hotplug
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handler and traffic will go through the virtio-net device. On the target
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system the primary device will be automatically plugged back and the
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net_failover module registers it again as the primary device.</p>
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</div></blockquote>
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</section>
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<section id="usage">
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<h2>Usage<a class="headerlink" href="#usage" title="Link to this heading"></a></h2>
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<blockquote>
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<div><p>The primary device can be hotplugged or be part of the startup configuration</p>
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<dl class="simple">
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<dt>-device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc, </dt><dd><p>bus=root2,failover=on</p>
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</dd>
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</dl>
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<p>With the parameter failover=on the VIRTIO_NET_F_STANDBY feature will be enabled.</p>
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<p>-device vfio-pci,host=5e:00.2,id=hostdev0,bus=root1,failover_pair_id=net1</p>
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<p>failover_pair_id references the id of the virtio-net standby device. This
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is only for pairing the devices within QEMU. The guest kernel module
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net_failover will match devices with identical MAC addresses.</p>
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</div></blockquote>
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</section>
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<section id="hotplug">
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<h2>Hotplug<a class="headerlink" href="#hotplug" title="Link to this heading"></a></h2>
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<blockquote>
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<div><p>Both primary and standby device can be hotplugged via the QEMU monitor. Note
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that if the virtio-net device is plugged first a warning will be issued that it
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couldn’t find the primary device.</p>
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</div></blockquote>
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</section>
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<section id="migration">
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<h2>Migration<a class="headerlink" href="#migration" title="Link to this heading"></a></h2>
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<blockquote>
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<div><p>A new migration state wait-unplug was added for this feature. If failover primary
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devices are present in the configuration, migration will go into this state.
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It will wait until the device unplug is completed in the guest and then move into
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active state. On the target system the primary devices will be automatically hotplugged
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when the feature bit was negotiated for the virtio-net standby device.</p>
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</div></blockquote>
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