69 lines
2.9 KiB
ReStructuredText
69 lines
2.9 KiB
ReStructuredText
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======================================
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QEMU virtio-net standby (net_failover)
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======================================
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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.
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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.
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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.
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Restrictions
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------------
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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.
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Usecase
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-------
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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.
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Usage
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-----
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The primary device can be hotplugged or be part of the startup configuration
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-device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc, \
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bus=root2,failover=on
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With the parameter failover=on the VIRTIO_NET_F_STANDBY feature will be enabled.
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-device vfio-pci,host=5e:00.2,id=hostdev0,bus=root1,failover_pair_id=net1
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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.
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Hotplug
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-------
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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.
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Migration
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---------
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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.
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