Using Open vSwitch with DPDK

Open vSwitch can use Intel(R) DPDK lib to operate entirely in userspace. This file explains how to install and use Open vSwitch in such a mode.

The DPDK support of Open vSwitch is considered experimental. It has not been thoroughly tested.

This version of Open vSwitch should be built manually with configure and make.

OVS needs a system with 1GB hugepages support.

Building and Installing:

Required: DPDK 2.2 Optional (if building with vhost-cuse): fuse, fuse-devel (libfuse-dev on Debian/Ubuntu)

  1. Configure build & install DPDK:

    1. Set $DPDK_DIR

      export DPDK_DIR=/usr/src/dpdk-2.2 cd $DPDK_DIR

    2. Update config/common_linuxapp so that DPDK generate single lib file. (modification also required for IVSHMEM build)

      CONFIG_RTE_BUILD_COMBINE_LIBS=y

      Then run make install to build and install the library. For default install without IVSHMEM:

      make install T=x86_64-native-linuxapp-gcc

      To include IVSHMEM (shared memory):

      make install T=x86_64-ivshmem-linuxapp-gcc

      For further details refer to http://dpdk.org/

  2. Configure & build the Linux kernel:

    Refer to intel-dpdk-getting-started-guide.pdf for understanding DPDK kernel requirement.

  3. Configure & build OVS:

    cd $(OVS_DIR)/ ./boot.sh ./configure --with-dpdk=$DPDK_BUILD [CFLAGS="-g -O2 -Wno-cast-align"] make

    Note: 'clang' users may specify the '-Wno-cast-align' flag to suppress DPDK cast-align warnings.

To have better performance one can enable aggressive compiler optimizations and use the special instructions(popcnt, crc32) that may not be available on all machines. Instead of typing make, type:

make CFLAGS='-O3 -march=native'

Refer to [INSTALL.userspace.md] for general requirements of building userspace OVS.

Using the DPDK with ovs-vswitchd:

  1. Setup system boot Add the following options to the kernel bootline:

    default_hugepagesz=1GB hugepagesz=1G hugepages=1

  2. Setup DPDK devices:

    DPDK devices can be setup using either the VFIO (for DPDK 1.7+) or UIO modules. UIO requires inserting an out of tree driver igb_uio.ko that is available in DPDK. Setup for both methods are described below.

  3. Mount the hugetable filesystem

    mount -t hugetlbfs -o pagesize=1G none /dev/hugepages

    Ref to http://www.dpdk.org/doc/quick-start for verifying DPDK setup.

  4. Follow the instructions in [INSTALL.md] to install only the userspace daemons and utilities (via 'make install').

    1. First time only db creation (or clearing):

      mkdir -p /usr/local/etc/openvswitch mkdir -p /usr/local/var/run/openvswitch rm /usr/local/etc/openvswitch/conf.db ovsdb-tool create /usr/local/etc/openvswitch/conf.db \ /usr/local/share/openvswitch/vswitch.ovsschema

    2. Start ovsdb-server

      ovsdb-server --remote=punix:/usr/local/var/run/openvswitch/db.sock \ --remote=db:Open_vSwitch,Open_vSwitch,manager_options \ --private-key=db:Open_vSwitch,SSL,private_key \ --certificate=Open_vSwitch,SSL,certificate \ --bootstrap-ca-cert=db:Open_vSwitch,SSL,ca_cert --pidfile --detach

    3. First time after db creation, initialize:

      ovs-vsctl --no-wait init

  5. Start vswitchd:

    DPDK configuration arguments can be passed to vswitchd via --dpdk argument. This needs to be first argument passed to vswitchd process. dpdk arg -c is ignored by ovs-dpdk, but it is a required parameter for dpdk initialization.

    export DB_SOCK=/usr/local/var/run/openvswitch/db.sock ovs-vswitchd --dpdk -c 0x1 -n 4 -- unix:$DB_SOCK --pidfile --detach

    If allocated more than one GB hugepage (as for IVSHMEM), set amount and use NUMA node 0 memory:

    ovs-vswitchd --dpdk -c 0x1 -n 4 --socket-mem 1024,0 \ -- unix:$DB_SOCK --pidfile --detach

  6. Add bridge & ports

    To use ovs-vswitchd with DPDK, create a bridge with datapath_type "netdev" in the configuration database. For example:

    ovs-vsctl add-br br0 -- set bridge br0 datapath_type=netdev

    Now you can add dpdk devices. OVS expects DPDK device names to start with "dpdk" and end with a portid. vswitchd should print (in the log file) the number of dpdk devices found.

    ovs-vsctl add-port br0 dpdk0 -- set Interface dpdk0 type=dpdk ovs-vsctl add-port br0 dpdk1 -- set Interface dpdk1 type=dpdk

    Once first DPDK port is added to vswitchd, it creates a Polling thread and polls dpdk device in continuous loop. Therefore CPU utilization for that thread is always 100%.

    Note: creating bonds of DPDK interfaces is slightly different to creating bonds of system interfaces. For DPDK, the interface type must be explicitly set, for example:

    ovs-vsctl add-bond br0 dpdkbond dpdk0 dpdk1 -- set Interface dpdk0 type=dpdk -- set Interface dpdk1 type=dpdk

  7. Add test flows

    Test flow script across NICs (assuming ovs in /usr/src/ovs): Execute script:

    ```

    ! /bin/sh

    Move to command directory

    cd /usr/src/ovs/utilities/

    Clear current flows

    ./ovs-ofctl del-flows br0

    Add flows between port 1 (dpdk0) to port 2 (dpdk1)

    ./ovs-ofctl add-flow br0 inport=1,action=output:2 ./ovs-ofctl add-flow br0 inport=2,action=output:1 ```

Performance Tuning:

  1. PMD affinitization

    A poll mode driver (pmd) thread handles the I/O of all DPDK interfaces assigned to it. A pmd thread will busy loop through the assigned port/rxq's polling for packets, switch the packets and send to a tx port if required. Typically, it is found that a pmd thread is CPU bound, meaning that the greater the CPU occupancy the pmd thread can get, the better the performance. To that end, it is good practice to ensure that a pmd thread has as many cycles on a core available to it as possible. This can be achieved by affinitizing the pmd thread with a core that has no other workload. See section 7 below for a description of how to isolate cores for this purpose also.

    The following command can be used to specify the affinity of the pmd thread(s).

    ovs-vsctl set Open_vSwitch . other_config:pmd-cpu-mask=<hex string>

    By setting a bit in the mask, a pmd thread is created and pinned to the corresponding CPU core. e.g. to run a pmd thread on core 1

    ovs-vsctl set Open_vSwitch . other_config:pmd-cpu-mask=2

    For more information, please refer to the Open_vSwitch TABLE section in

    man ovs-vswitchd.conf.db

    Note, that a pmd thread on a NUMA node is only created if there is at least one DPDK interface from that NUMA node added to OVS.

  2. Multiple poll mode driver threads

    With pmd multi-threading support, OVS creates one pmd thread for each NUMA node by default. However, it can be seen that in cases where there are multiple ports/rxq's producing traffic, performance can be improved by creating multiple pmd threads running on separate cores. These pmd threads can then share the workload by each being responsible for different ports/rxq's. Assignment of ports/rxq's to pmd threads is done automatically.

    The following command can be used to specify the affinity of the pmd threads.

    ovs-vsctl set Open_vSwitch . other_config:pmd-cpu-mask=<hex string>

    A set bit in the mask means a pmd thread is created and pinned to the corresponding CPU core. e.g. to run pmd threads on core 1 and 2

    ovs-vsctl set Open_vSwitch . other_config:pmd-cpu-mask=6

    For more information, please refer to the Open_vSwitch TABLE section in

    man ovs-vswitchd.conf.db

    For example, when using dpdk and dpdkvhostuser ports in a bi-directional VM loopback as shown below, spreading the workload over 2 or 4 pmd threads shows significant improvements as there will be more total CPU occupancy available.

    NIC port0 <-> OVS <-> VM <-> OVS <-> NIC port 1

    The OVS log can be checked to confirm that the port/rxq assignment to pmd threads is as required. This can also be checked with the following commands:

    top -H taskset -p <pid_of_pmd>

    To understand where most of the pmd thread time is spent and whether the caches are being utilized, these commands can be used:

    ```

    Clear previous stats

    ovs-appctl dpif-netdev/pmd-stats-clear

    Check current stats

    ovs-appctl dpif-netdev/pmd-stats-show ```

  3. DPDK port Rx Queues

    ovs-vsctl set Open_vSwitch . other_config:n-dpdk-rxqs=<integer>

    The command above sets the number of rx queues for each DPDK interface. The rx queues are assigned to pmd threads on the same NUMA node in a round-robin fashion. For more information, please refer to the Open_vSwitch TABLE section in

    man ovs-vswitchd.conf.db

  4. Exact Match Cache

    Each pmd thread contains one EMC. After initial flow setup in the datapath, the EMC contains a single table and provides the lowest level (fastest) switching for DPDK ports. If there is a miss in the EMC then the next level where switching will occur is the datapath classifier. Missing in the EMC and looking up in the datapath classifier incurs a significant performance penalty. If lookup misses occur in the EMC because it is too small to handle the number of flows, its size can be increased. The EMC size can be modified by editing the define EMFLOWHASH_SHIFT in lib/dpif-netdev.c.

    As mentioned above an EMC is per pmd thread. So an alternative way of increasing the aggregate amount of possible flow entries in EMC and avoiding datapath classifier lookups is to have multiple pmd threads running. This can be done as described in section 2.

  5. Compiler options

    The default compiler optimization level is '-O2'. Changing this to more aggressive compiler optimizations such as '-O3' or '-Ofast -march=native' with gcc can produce performance gains.

  6. Simultaneous Multithreading (SMT)

    With SMT enabled, one physical core appears as two logical cores which can improve performance.

    SMT can be utilized to add additional pmd threads without consuming additional physical cores. Additional pmd threads may be added in the same manner as described in section 2. If trying to minimize the use of physical cores for pmd threads, care must be taken to set the correct bits in the pmd-cpu-mask to ensure that the pmd threads are pinned to SMT siblings.

    For example, when using 2x 10 core processors in a dual socket system with HT enabled, /proc/cpuinfo will report 40 logical cores. To use two logical cores which share the same physical core for pmd threads, the following command can be used to identify a pair of logical cores.

    cat /sys/devices/system/cpu/cpuN/topology/thread_siblings_list

    where N is the logical core number. In this example, it would show that cores 1 and 21 share the same physical core. The pmd-cpu-mask to enable two pmd threads running on these two logical cores (one physical core) is.

    ovs-vsctl set Open_vSwitch . other_config:pmd-cpu-mask=100002

    Note that SMT is enabled by the Hyper-Threading section in the BIOS, and as such will apply to the whole system. So the impact of enabling/disabling it for the whole system should be considered e.g. If workloads on the system can scale across multiple cores, SMT may very beneficial. However, if they do not and perform best on a single physical core, SMT may not be beneficial.

  7. The isolcpus kernel boot parameter

    isolcpus can be used on the kernel bootline to isolate cores from the kernel scheduler and hence dedicate them to OVS or other packet forwarding related workloads. For example a Linux kernel boot-line could be:

    'GRUBCMDLINELINUXDEFAULT="quiet hugepagesz=1G hugepages=4 defaulthugepagesz=1G 'intel_iommu=off' isolcpus=1-19"'

  8. NUMA/Cluster On Die

    Ideally inter NUMA datapaths should be avoided where possible as packets will go across QPI and there may be a slight performance penalty when compared with intra NUMA datapaths. On Intel Xeon Processor E5 v3, Cluster On Die is introduced on models that have 10 cores or more. This makes it possible to logically split a socket into two NUMA regions and again it is preferred where possible to keep critical datapaths within the one cluster.

    It is good practice to ensure that threads that are in the datapath are pinned to cores in the same NUMA area. e.g. pmd threads and QEMU vCPUs responsible for forwarding.

  9. Rx Mergeable buffers

    Rx Mergeable buffers is a virtio feature that allows chaining of multiple virtio descriptors to handle large packet sizes. As such, large packets are handled by reserving and chaining multiple free descriptors together. Mergeable buffer support is negotiated between the virtio driver and virtio device and is supported by the DPDK vhost library. This behavior is typically supported and enabled by default, however in the case where the user knows that rx mergeable buffers are not needed i.e. jumbo frames are not needed, it can be forced off by adding mrg_rxbuf=off to the QEMU command line options. By not reserving multiple chains of descriptors it will make more individual virtio descriptors available for rx to the guest using dpdkvhost ports and this can improve performance.

  10. Packet processing in the guest

    It is good practice whether simply forwarding packets from one interface to another or more complex packet processing in the guest, to ensure that the thread performing this work has as much CPU occupancy as possible. For example when the DPDK sample application testpmd is used to forward packets in the guest, multiple QEMU vCPU threads can be created. Taskset can then be used to affinitize the vCPU thread responsible for forwarding to a dedicated core not used for other general processing on the host system.

  11. DPDK virtio pmd in the guest

    dpdkvhostcuse or dpdkvhostuser ports can be used to accelerate the path to the guest using the DPDK vhost library. This library is compatible with virtio-net drivers in the guest but significantly better performance can be observed when using the DPDK virtio pmd driver in the guest. The DPDK testpmd application can be used in the guest as an example application that forwards packet from one DPDK vhost port to another. An example of running testpmd in the guest can be seen here.

    ./testpmd -c 0x3 -n 4 --socket-mem 512 -- --burst=64 -i --txqflags=0xf00 --disable-hw-vlan --forward-mode=io --auto-start

    See below information on dpdkvhostcuse and dpdkvhostuser ports. See [DPDK Docs] for more information on testpmd.

DPDK Rings :

Following the steps above to create a bridge, you can now add dpdk rings as a port to the vswitch. OVS will expect the DPDK ring device name to start with dpdkr and end with a portid.

ovs-vsctl add-port br0 dpdkr0 -- set Interface dpdkr0 type=dpdkr

DPDK rings client test application

Included in the test directory is a sample DPDK application for testing the rings. This is from the base dpdk directory and modified to work with the ring naming used within ovs.

location tests/ovs_client

To run the client :

cd /usr/src/ovs/tests/ ovsclient -c 1 -n 4 --proc-type=secondary -- -n "port id you gave dpdkr"

In the case of the dpdkr example above the "port id you gave dpdkr" is 0.

It is essential to have --proc-type=secondary

The application simply receives an mbuf on the receive queue of the ethernet ring and then places that same mbuf on the transmit ring of the ethernet ring. It is a trivial loopback application.

DPDK rings in VM (IVSHMEM shared memory communications)

In addition to executing the client in the host, you can execute it within a guest VM. To do so you will need a patched qemu. You can download the patch and getting started guide at :

https://01.org/packet-processing/downloads

A general rule of thumb for better performance is that the client application should not be assigned the same dpdk core mask "-c" as the vswitchd.

DPDK vhost:

DPDK 2.2 supports two types of vhost:

  1. vhost-user
  2. vhost-cuse

Whatever type of vhost is enabled in the DPDK build specified, is the type that will be enabled in OVS. By default, vhost-user is enabled in DPDK. Therefore, unless vhost-cuse has been enabled in DPDK, vhost-user ports will be enabled in OVS. Please note that support for vhost-cuse is intended to be deprecated in OVS in a future release.

DPDK vhost-user:

The following sections describe the use of vhost-user 'dpdkvhostuser' ports with OVS.

DPDK vhost-user Prerequisites:

  1. DPDK 2.2 with vhost support enabled as documented in the "Building and Installing section"

  2. QEMU version v2.1.0+

    QEMU v2.1.0 will suffice, but it is recommended to use v2.2.0 if providing your VM with memory greater than 1GB due to potential issues with memory mapping larger areas.

Adding DPDK vhost-user ports to the Switch:

Following the steps above to create a bridge, you can now add DPDK vhost-user as a port to the vswitch. Unlike DPDK ring ports, DPDK vhost-user ports can have arbitrary names, except that forward and backward slashes are prohibited in the names.

DPDK vhost-user VM configuration:

Follow the steps below to attach vhost-user port(s) to a VM.

  1. Configure sockets. Pass the following parameters to QEMU to attach a vhost-user device:

    -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

  2. Configure huge pages. 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 paramters to QEMU:

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

  3. Optional: Enable multiqueue support QEMU needs to be configured with multiple queues and the number queues must be less or equal to Open vSwitch other_config:n-dpdk-rxqs. The $q below is the number of queues. The $v is the number of vectors, which is '$q x 2 + 2'.

    -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

    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>

    A note on the command above:

    -L: Changes the numbers of channels of the specified network device

    combined: Changes the number of multi-purpose channels.

DPDK vhost-cuse:

The following sections describe the use of vhost-cuse 'dpdkvhostcuse' ports with OVS.

DPDK vhost-cuse Prerequisites:

  1. DPDK 2.2 with vhost support enabled as documented in the "Building and Installing section" As an additional step, you must enable vhost-cuse in DPDK by setting the following additional flag in config/common_linuxapp:

    CONFIG_RTE_LIBRTE_VHOST_USER=n

    Following this, rebuild DPDK as per the instructions in the "Building and Installing" section. Finally, rebuild OVS as per step 3 in the "Building and Installing" section - OVS will detect that DPDK has vhost-cuse libraries compiled and in turn will enable support for it in the switch and disable vhost-user support.

  2. Insert the Cuse module:

    modprobe cuse

  3. Build and insert the eventfd_link module:

    cd $DPDK_DIR/lib/librte_vhost/eventfd_link/ make insmod $DPDK_DIR/lib/librte_vhost/eventfd_link.ko

  4. QEMU version v2.1.0+

    vhost-cuse will work with QEMU v2.1.0 and above, however it is recommended to use v2.2.0 if providing your VM with memory greater than 1GB due to potential issues with memory mapping larger areas. Note: QEMU v1.6.2 will also work, with slightly different command line parameters, which are specified later in this document.

Adding DPDK vhost-cuse ports to the Switch:

Following the steps above to create a bridge, you can now add DPDK vhost-cuse as a port to the vswitch. Unlike DPDK ring ports, DPDK vhost-cuse ports can have arbitrary names.

DPDK vhost-cuse VM configuration:

vhost-cuse ports use a Linux* character device to communicate with QEMU. By default it is set to /dev/vhost-net. It is possible to reuse this standard device for DPDK vhost, which makes setup a little simpler but it is better practice to specify an alternative character device in order to avoid any conflicts if kernel vhost is to be used in parallel.

  1. This step is only needed if using an alternative character device.

    The new character device filename must be specified on the vswitchd commandline:

    `./vswitchd/ovs-vswitchd --dpdk --cuse_dev_name my-vhost-net -c 0x1 ...`
    

    Note that the --cuse_dev_name argument and associated string must be the first arguments after --dpdk and come before the EAL arguments. In the example above, the character device to be used will be /dev/my-vhost-net.

  2. This step is only needed if reusing the standard character device. It will conflict with the kernel vhost character device so the user must first remove it.

    rm -rf /dev/vhost-net

3a. Configure virtio-net adaptors: The following parameters must be passed to the QEMU binary:

 ```
 -netdev tap,id=<id>,script=no,downscript=no,ifname=<name>,vhost=on
 -device virtio-net-pci,netdev=net1,mac=<mac>
 ```

 Repeat the above parameters for multiple devices.

 The DPDK vhost library will negiotiate its own features, so they
 need not be passed in as command line params. Note that as offloads are
 disabled this is the equivalent of setting:

 `csum=off,gso=off,guest_tso4=off,guest_tso6=off,guest_ecn=off`

3b. If using an alternative character device. It must be also explicitly passed to QEMU using the vhostfd argument:

 ```
 -netdev tap,id=<id>,script=no,downscript=no,ifname=<name>,vhost=on,
 vhostfd=<open_fd>
 -device virtio-net-pci,netdev=net1,mac=<mac>
 ```

 The open file descriptor must be passed to QEMU running as a child
 process. This could be done with a simple python script.

   ```
   #!/usr/bin/python
   fd = os.open("/dev/usvhost", os.O_RDWR)
   subprocess.call("qemu-system-x86_64 .... -netdev tap,id=vhostnet0,\
                    vhost=on,vhostfd=" + fd +"...", shell=True)

Alternatively the qemu-wrap.py script can be used to automate the requirements specified above and can be used in conjunction with libvirt if desired. See the "DPDK vhost VM configuration with QEMU wrapper" section below.

  1. Configure huge pages: QEMU must allocate the VM's memory on hugetlbfs. Vhost ports access a virtio-net device's virtual rings and packet buffers mapping the VM's physical memory on hugetlbfs. To enable vhost-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

    Note: For use with an earlier QEMU version such as v1.6.2, use the following to configure hugepages instead:

    -mem-path /dev/hugepages -mem-prealloc

DPDK vhost-cuse VM configuration with QEMU wrapper:

The QEMU wrapper script automatically detects and calls QEMU with the necessary parameters. It performs the following actions:

Before use, you must edit the configuration parameters section of the script to point to the correct emulator location and set additional settings. Of these settings, emul_path and us_vhost_path must be set. All other settings are optional.

To use directly from the command line simply pass the wrapper some of the QEMU parameters: it will configure the rest. For example:

qemu-wrap.py -cpu host -boot c -hda <disk image> -m 4096 -smp 4 --enable-kvm -nographic -vnc none -net none -netdev tap,id=net1, script=no,downscript=no,ifname=if1,vhost=on -device virtio-net-pci, netdev=net1,mac=00:00:00:00:00:01

DPDK vhost-cuse VM configuration with libvirt:

If you are using libvirt, you must enable libvirt to access the character device by adding it to controllers cgroup for libvirtd using the following steps.

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

    ```
    1) clear_emulator_capabilities = 0
    2) user = "root"
    3) group = "root"
    4) cgroup_device_acl = [
           "/dev/null", "/dev/full", "/dev/zero",
           "/dev/random", "/dev/urandom",
           "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
           "/dev/rtc", "/dev/hpet", "/dev/net/tun",
           "/dev/<my-vhost-device>",
           "/dev/hugepages"]
    ```

    <my-vhost-device> refers to "vhost-net" if using the `/dev/vhost-net`
    device. If you have specificed a different name on the ovs-vswitchd
    commandline using the "--cuse_dev_name" parameter, please specify that
    filename instead.

 2. Disable SELinux or set to permissive mode

 3. Restart the libvirtd process
    For example, on Fedora:

      `systemctl restart libvirtd.service`

After successfully editing the configuration, you may launch your vhost-enabled VM. The XML describing the VM can be configured like so within the section:

 1. Set up shared hugepages:

 ```
 <qemu:arg value='-object'/>
 <qemu:arg value='memory-backend-file,id=mem,size=4096M,mem-path=/dev/hugepages,share=on'/>
 <qemu:arg value='-numa'/>
 <qemu:arg value='node,memdev=mem'/>
 <qemu:arg value='-mem-prealloc'/>
 ```

 2. Set up your tap devices:

 ```
 <qemu:arg value='-netdev'/>
 <qemu:arg value='type=tap,id=net1,script=no,downscript=no,ifname=vhost0,vhost=on'/>
 <qemu:arg value='-device'/>
 <qemu:arg value='virtio-net-pci,netdev=net1,mac=00:00:00:00:00:01'/>
 ```

 Repeat for as many devices as are desired, modifying the id, ifname
 and mac as necessary.

 Again, if you are using an alternative character device (other than
 `/dev/vhost-net`), please specify the file descriptor like so:

 `<qemu:arg value='type=tap,id=net3,script=no,downscript=no,ifname=vhost0,vhost=on,vhostfd=<open_fd>'/>`

 Where <open_fd> refers to the open file descriptor of the character device.
 Instructions of how to retrieve the file descriptor can be found in the
 "DPDK vhost VM configuration" section.
 Alternatively, the process is automated with the qemu-wrap.py script,
 detailed in the next section.

Now you may launch your VM using virt-manager, or like so:

`virsh create my_vhost_vm.xml`

DPDK vhost-cuse VM configuration with libvirt and QEMU wrapper:

To use the qemu-wrapper script in conjuntion with libvirt, follow the steps in the previous section before proceeding with the following steps:

  1. Place qemu-wrap.py in libvirtd's binary search PATH ($PATH) Ideally in the same directory that the QEMU binary is located.

  2. Ensure that the script has the same owner/group and file permissions as the QEMU binary.

  3. Update the VM xml file using "virsh edit VM.xml"

    1. Set the VM to use the launch script. Set the emulator path contained in the <emulator><emulator/> tags. For example, replace:

      `<emulator>/usr/bin/qemu-kvm<emulator/>`
      
      
      with:
      
      
      `<emulator>/usr/bin/qemu-wrap.py<emulator/>`
      
  4. Edit the Configuration Parameters section of the script to point to the correct emulator location and set any additional options. If you are using a alternative character device name, please set "usvhostpath" to the location of that device. The script will automatically detect and insert the correct "vhostfd" value in the QEMU command line arguments.

  5. Use virt-manager to launch the VM

Running ovs-vswitchd with DPDK backend inside a VM

Please note that additional configuration is required if you want to run ovs-vswitchd with DPDK backend inside a QEMU virtual machine. Ovs-vswitchd creates separate DPDK TX queues for each CPU core available. This operation fails inside QEMU virtual machine because, by default, VirtIO NIC provided to the guest is configured to support only single TX queue and single RX queue. To change this behavior, you need to turn on 'mq' (multiqueue) property of all virtio-net-pci devices emulated by QEMU and used by DPDK. You may do it manually (by changing QEMU command line) or, if you use Libvirt, by adding the following string:

<driver name='vhost' queues='N'/>

to sections of all network devices used by DPDK. Parameter 'N' determines how many queues can be used by the guest.

Restrictions:

Bug Reporting:

Please report problems to bugs@openvswitch.org.