DPDK vHost User Ports

The DPDK datapath provides DPDK-backed vHost user ports as a primary way to interact with guests. For more information on vHost User, refer to the QEMU documentation on same.


To use any DPDK-backed interface, you must ensure your bridge is configured correctly. For more information, refer to DPDK Bridges.

Quick Example

This example demonstrates how to add two dpdkvhostuserclient ports to an existing bridge called br0:

$ ovs-vsctl add-port br0 dpdkvhostclient0 \
    -- set Interface dpdkvhostclient0 type=dpdkvhostuserclient \
$ ovs-vsctl add-port br0 dpdkvhostclient1 \
    -- set Interface dpdkvhostclient1 type=dpdkvhostuserclient \

For the above examples to work, an appropriate server socket must be created at the paths specified (/tmp/dpdkvhostclient0 and /tmp/dpdkvhostclient1). These sockets can be created with QEMU; see the vhost-user client section for details.

vhost-user vs. vhost-user-client

Open vSwitch provides two types of vHost User ports:

  • vhost-user (dpdkvhostuser)
  • vhost-user-client (dpdkvhostuserclient)

vHost User uses a client-server model. The server creates/manages/destroys the vHost User sockets, and the client connects to the server. Depending on which port type you use, dpdkvhostuser or dpdkvhostuserclient, a different configuration of the client-server model is used.

For vhost-user ports, Open vSwitch acts as the server and QEMU the client. This means if OVS dies, all VMs must be restarted. On the other hand, for vhost-user-client ports, OVS acts as the client and QEMU the server. This means OVS can die and be restarted without issue, and it is also possible to restart an instance itself. For this reason, vhost-user-client ports are the preferred type for all known use cases; the only limitation is that vhost-user client mode ports require QEMU version 2.7. Ports of type vhost-user are currently deprecated and will be removed in a future release.



Use of vhost-user ports requires QEMU >= 2.2; vhost-user ports are deprecated.

To use vhost-user ports, you must first add said ports to the switch. DPDK vhost-user ports can have arbitrary names with the exception of forward and backward slashes, which are prohibited. For vhost-user, the port type is dpdkvhostuser:

$ ovs-vsctl add-port br0 vhost-user-1 -- set Interface vhost-user-1 \

This action creates a socket located at /usr/local/var/run/openvswitch/vhost-user-1, which you must provide to your VM on the QEMU command line.


If you wish for the vhost-user sockets to be created in a sub-directory of /usr/local/var/run/openvswitch, you may specify this directory in the ovsdb like so:

$ ovs-vsctl --no-wait \
    set Open_vSwitch . other_config:vhost-sock-dir=subdir

Once the vhost-user ports have been added to the switch, they must be added to the guest. There are two ways to do this: using QEMU directly, or using libvirt.


IOMMU is not supported with vhost-user ports.

Adding vhost-user ports to the guest (QEMU)

To begin, you must attach the vhost-user device sockets to the guest. To do this, you must pass the following parameters to QEMU:

-chardev socket,id=char1,path=/usr/local/var/run/openvswitch/vhost-user-1
-netdev type=vhost-user,id=mynet1,chardev=char1,vhostforce
-device virtio-net-pci,mac=00:00:00:00:00:01,netdev=mynet1

where vhost-user-1 is the name of the vhost-user port added to the switch.

Repeat the above parameters for multiple devices, changing the chardev path and id as necessary. Note that a separate and different chardev path needs to be specified for each vhost-user device. For example you have a second vhost-user port named vhost-user-2, you append your QEMU command line with an additional set of parameters:

-chardev socket,id=char2,path=/usr/local/var/run/openvswitch/vhost-user-2
-netdev type=vhost-user,id=mynet2,chardev=char2,vhostforce
-device virtio-net-pci,mac=00:00:00:00:00:02,netdev=mynet2

In addition, QEMU must allocate the VM’s memory on hugetlbfs. vhost-user ports access a virtio-net device’s virtual rings and packet buffers mapping the VM’s physical memory on hugetlbfs. To enable vhost-user ports to map the VM’s memory into their process address space, pass the following parameters to QEMU:

-object memory-backend-file,id=mem,size=4096M,mem-path=/dev/hugepages,share=on
-numa node,memdev=mem -mem-prealloc

Finally, you may wish to enable multiqueue support. This is optional but, should you wish to enable it, run:

-chardev socket,id=char2,path=/usr/local/var/run/openvswitch/vhost-user-2
-netdev type=vhost-user,id=mynet2,chardev=char2,vhostforce,queues=$q
-device virtio-net-pci,mac=00:00:00:00:00:02,netdev=mynet2,mq=on,vectors=$v


The number of queues
The number of vectors, which is $q * 2 + 2

The vhost-user interface will be automatically reconfigured with required number of Rx and Tx queues after connection of virtio device. Manual configuration of n_rxq is not supported because OVS will work properly only if n_rxq will match number of queues configured in QEMU.

A least two PMDs should be configured for the vswitch when using multiqueue. Using a single PMD will cause traffic to be enqueued to the same vhost queue rather than being distributed among different vhost queues for a vhost-user interface.

If traffic destined for a VM configured with multiqueue arrives to the vswitch via a physical DPDK port, then the number of Rx queues should also be set to at least two for that physical DPDK port. This is required to increase the probability that a different PMD will handle the multiqueue transmission to the guest using a different vhost queue.

If one wishes to use multiple queues for an interface in the guest, the driver in the guest operating system must be configured to do so. It is recommended that the number of queues configured be equal to $q.

For example, this can be done for the Linux kernel virtio-net driver with:

$ ethtool -L <DEV> combined <$q>


Changes the numbers of channels of the specified network device
Changes the number of multi-purpose channels.

Adding vhost-user ports to the guest (libvirt)

To begin, you must change the user and group that qemu runs under, and restart libvirtd.

  • In /etc/libvirt/qemu.conf add/edit the following lines:

    user = "root"
    group = "root"
  • Finally, restart the libvirtd process, For example, on Fedora:

    $ systemctl restart libvirtd.service

Once complete, instantiate the VM. A sample XML configuration file is provided at the end of this file. Save this file, then create a VM using this file:

$ virsh create demovm.xml

Once created, you can connect to the guest console:

$ virsh console demovm

The demovm xml configuration is aimed at achieving out of box performance on VM. These enhancements include:

  • The vcpus are pinned to the cores of the CPU socket 0 using vcpupin.
  • Configure NUMA cell and memory shared using memAccess='shared'.
  • Disable mrg_rxbuf='off'

Refer to the libvirt documentation for more information.



Use of vhost-user ports requires QEMU >= 2.7

To use vhost-user-client ports, you must first add said ports to the switch. Like DPDK vhost-user ports, DPDK vhost-user-client ports can have mostly arbitrary names. However, the name given to the port does not govern the name of the socket device. Instead, this must be configured by the user by way of a vhost-server-path option. For vhost-user-client, the port type is dpdkvhostuserclient:

$ VHOST_USER_SOCKET_PATH=/path/to/socket
$ ovs-vsctl add-port br0 vhost-client-1 \
    -- set Interface vhost-client-1 type=dpdkvhostuserclient \

Once the vhost-user-client ports have been added to the switch, they must be added to the guest. Like vhost-user ports, there are two ways to do this: using QEMU directly, or using libvirt. Only the QEMU case is covered here.

Adding vhost-user-client ports to the guest (QEMU)

Attach the vhost-user device sockets to the guest. To do this, you must pass the following parameters to QEMU:

-chardev socket,id=char1,path=$VHOST_USER_SOCKET_PATH,server
-netdev type=vhost-user,id=mynet1,chardev=char1,vhostforce
-device virtio-net-pci,mac=00:00:00:00:00:01,netdev=mynet1

where vhost-user-1 is the name of the vhost-user port added to the switch.

If the corresponding dpdkvhostuserclient port has not yet been configured in OVS with vhost-server-path=/path/to/socket, QEMU will print a log similar to the following:

QEMU waiting for connection on: disconnected:unix:/path/to/socket,server

QEMU will wait until the port is created sucessfully in OVS to boot the VM. One benefit of using this mode is the ability for vHost ports to ‘reconnect’ in event of the switch crashing or being brought down. Once it is brought back up, the vHost ports will reconnect automatically and normal service will resume.

vhost-user-client IOMMU Support

vhost IOMMU is a feature which restricts the vhost memory that a virtio device can access, and as such is useful in deployments in which security is a concern.

IOMMU support may be enabled via a global config value, `vhost-iommu-support`. Setting this to true enables vhost IOMMU support for all vhost ports when/where available:

$ ovs-vsctl set Open_vSwitch . other_config:vhost-iommu-support=true

The default value is false.


Changing this value requires restarting the daemon.


Enabling the IOMMU feature also enables the vhost user reply-ack protocol; this is known to work on QEMU v2.10.0, but is buggy on older versions (2.7.0 - 2.9.0, inclusive). Consequently, the IOMMU feature is disabled by default (and should remain so if using the aforementioned versions of QEMU). Starting with QEMU v2.9.1, vhost-iommu-support can safely be enabled, even without having an IOMMU device, with no performance penalty.

DPDK in the Guest

The DPDK testpmd application can be run in guest VMs for high speed packet forwarding between vhostuser ports. DPDK and testpmd application has to be compiled on the guest VM. Below are the steps for setting up the testpmd application in the VM.


Support for DPDK in the guest requires QEMU >= 2.2

To begin, instantiate a guest as described in vhost-user or vhost-user-client. Once started, connect to the VM, download the DPDK sources to VM and build DPDK:

$ cd /root/dpdk/
$ wget http://fast.dpdk.org/rel/dpdk-17.11.3.tar.xz
$ tar xf dpdk-17.11.3.tar.xz
$ export DPDK_DIR=/root/dpdk/dpdk-stable-17.11.3
$ export DPDK_TARGET=x86_64-native-linuxapp-gcc
$ cd $DPDK_DIR
$ make install T=$DPDK_TARGET DESTDIR=install

Build the test-pmd application:

$ cd app/test-pmd
$ export RTE_SDK=$DPDK_DIR
$ make

Setup huge pages and DPDK devices using UIO:

$ sysctl vm.nr_hugepages=1024
$ mkdir -p /dev/hugepages
$ mount -t hugetlbfs hugetlbfs /dev/hugepages  # only if not already mounted
$ modprobe uio
$ insmod $DPDK_BUILD/kmod/igb_uio.ko
$ $DPDK_DIR/usertools/dpdk-devbind.py --status
$ $DPDK_DIR/usertools/dpdk-devbind.py -b igb_uio 00:03.0 00:04.0


vhost ports pci ids can be retrieved using:

lspci | grep Ethernet

Finally, start the application:


Sample XML

<domain type='kvm'>
  <memory unit='KiB'>4194304</memory>
  <currentMemory unit='KiB'>4194304</currentMemory>
      <page size='2' unit='M' nodeset='0'/>
  <vcpu placement='static'>2</vcpu>
    <vcpupin vcpu='0' cpuset='4'/>
    <vcpupin vcpu='1' cpuset='5'/>
    <emulatorpin cpuset='4,5'/>
    <type arch='x86_64' machine='pc'>hvm</type>
    <boot dev='hd'/>
  <cpu mode='host-model'>
    <model fallback='allow'/>
    <topology sockets='2' cores='1' threads='1'/>
      <cell id='0' cpus='0-1' memory='4194304' unit='KiB' memAccess='shared'/>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/root/CentOS7_x86_64.qcow2'/>
      <target dev='vda' bus='virtio'/>
    <interface type='vhostuser'>
      <mac address='00:00:00:00:00:01'/>
      <source type='unix' path='/usr/local/var/run/openvswitch/dpdkvhostuser0' mode='client'/>
       <model type='virtio'/>
      <driver queues='2'>
        <host mrg_rxbuf='on'/>
    <interface type='vhostuser'>
      <mac address='00:00:00:00:00:02'/>
      <source type='unix' path='/usr/local/var/run/openvswitch/dpdkvhostuser1' mode='client'/>
      <model type='virtio'/>
      <driver queues='2'>
        <host mrg_rxbuf='on'/>
    <serial type='pty'>
      <target port='0'/>
    <console type='pty'>
      <target type='serial' port='0'/>

Jumbo Frames

DPDK vHost User ports can be configured to use Jumbo Frames. For more information, refer to Jumbo Frames.

vhost-user Dequeue Zero Copy (experimental)

Normally when dequeuing a packet from a vHost User device, a memcpy operation must be used to copy that packet from guest address space to host address space. This memcpy can be removed by enabling dequeue zero-copy like so:

$ ovs-vsctl add-port br0 dpdkvhostuserclient0 -- set Interface \
    dpdkvhostuserclient0 type=dpdkvhostuserclient \
    options:vhost-server-path=/tmp/dpdkvhostclient0 \

With this feature enabled, a reference (pointer) to the packet is passed to the host, instead of a copy of the packet. Removing this memcpy can give a performance improvement for some use cases, for example switching large packets between different VMs. However additional packet loss may be observed.

Note that the feature is disabled by default and must be explicitly enabled by setting the dq-zero-copy option to true while specifying the vhost-server-path option as above. If you wish to split out the command into multiple commands as below, ensure dq-zero-copy is set before vhost-server-path:

$ ovs-vsctl set Interface dpdkvhostuserclient0 options:dq-zero-copy=true
$ ovs-vsctl set Interface dpdkvhostuserclient0 \

The feature is only available to dpdkvhostuserclient port types.

A limitation exists whereby if packets from a vHost port with dq-zero-copy=true are destined for a dpdk type port, the number of tx descriptors (n_txq_desc) for that port must be reduced to a smaller number, 128 being the recommended value. This can be achieved by issuing the following command:

$ ovs-vsctl set Interface dpdkport options:n_txq_desc=128

Note: The sum of the tx descriptors of all dpdk ports the VM will send to should not exceed 128. For example, in case of a bond over two physical ports in balance-tcp mode, one must divide 128 by the number of links in the bond.

Refer to DPDK Physical Port Queue Sizes for more information.

The reason for this limitation is due to how the zero copy functionality is implemented. The vHost device’s ‘tx used vring’, a virtio structure used for tracking used ie. sent descriptors, will only be updated when the NIC frees the corresponding mbuf. If we don’t free the mbufs frequently enough, that vring will be starved and packets will no longer be processed. One way to ensure we don’t encounter this scenario, is to configure n_txq_desc to a small enough number such that the ‘mbuf free threshold’ for the NIC will be hit more often and thus free mbufs more frequently. The value of 128 is suggested, but values of 64 and 256 have been tested and verified to work too, with differing performance characteristics. A value of 512 can be used too, if the virtio queue size in the guest is increased to 1024 (available to configure in QEMU versions v2.10 and greater). This value can be set like so:

$ qemu-system-x86_64 ... -chardev socket,id=char1,path=<sockpath>,server
  -netdev type=vhost-user,id=mynet1,chardev=char1,vhostforce
  -device virtio-net-pci,mac=00:00:00:00:00:01,netdev=mynet1,

Because of this limitation, this feature is considered ‘experimental’.

The feature currently does not fully work with QEMU >= v2.7 due to a bug in DPDK which will be addressed in an upcoming release. The patch to fix this issue can be found on Patchwork

Further information can be found in the DPDK documentation