35. Vhost Sample Application

The vhost sample application demonstrates integration of the Data Plane Development Kit (DPDK) with the Linux* KVM hypervisor by implementing the vhost-net offload API. The sample application performs simple packet switching between virtual machines based on Media Access Control (MAC) address or Virtual Local Area Network (VLAN) tag. The splitting of Ethernet traffic from an external switch is performed in hardware by the Virtual Machine Device Queues (VMDQ) and Data Center Bridging (DCB) features of the IntelĀ® 82599 10 Gigabit Ethernet Controller.

35.1. Testing steps

This section shows the steps how to test a typical PVP case with this vhost-switch sample, whereas packets are received from the physical NIC port first and enqueued to the VM’s Rx queue. Through the guest testpmd’s default forwarding mode (io forward), those packets will be put into the Tx queue. The vhost-switch example, in turn, gets the packets and puts back to the same physical NIC port.

35.1.1. Build

To compile the sample application see Compiling the Sample Applications.

The application is located in the vhost sub-directory.


In this example, you need build DPDK both on the host and inside guest.

35.1.2. Start the vswitch example

./vhost-switch -l 0-3 -n 4 --socket-mem 1024  \
     -- --socket-file /tmp/sock0 --client \

Check the Parameters section for the explanations on what do those parameters mean.

35.1.3. Start the VM

qemu-system-x86_64 -machine accel=kvm -cpu host \
    -m $mem -object memory-backend-file,id=mem,size=$mem,mem-path=/dev/hugepages,share=on \
            -mem-prealloc -numa node,memdev=mem \
    -chardev socket,id=char1,path=/tmp/sock0,server \
    -netdev type=vhost-user,id=hostnet1,chardev=char1  \
    -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:00:00:14 \


For basic vhost-user support, QEMU 2.2 (or above) is required. For some specific features, a higher version might be need. Such as QEMU 2.7 (or above) for the reconnect feature.

35.1.4. Run testpmd inside guest

Make sure you have DPDK built inside the guest. Also make sure the corresponding virtio-net PCI device is bond to a uio driver, which could be done by:

modprobe uio_pci_generic
$RTE_SDK/usertools/dpdk-devbind.py -b=uio_pci_generic 0000:00:04.0

Then start testpmd for packet forwarding testing.

./x86_64-native-gcc/app/testpmd -l 0-1 -- -i
> start tx_first

35.2. Inject packets

While a virtio-net is connected to vhost-switch, a VLAN tag starts with 1000 is assigned to it. So make sure configure your packet generator with the right MAC and VLAN tag, you should be able to see following log from the vhost-switch console. It means you get it work:

VHOST_DATA: (0) mac 52:54:00:00:00:14 and vlan 1000 registered

35.3. Parameters

–socket-file path Specifies the vhost-user socket file path.

–client DPDK vhost-user will act as the client mode when such option is given. In the client mode, QEMU will create the socket file. Otherwise, DPDK will create it. Put simply, it’s the server to create the socket file.

–vm2vm mode The vm2vm parameter sets the mode of packet switching between guests in the host.

  • 0 disables vm2vm, impling that VM’s packets will always go to the NIC port.
  • 1 means a normal mac lookup packet routing.
  • 2 means hardware mode packet forwarding between guests, it allows packets go to the NIC port, hardware L2 switch will determine which guest the packet should forward to or need send to external, which bases on the packet destination MAC address and VLAN tag.

–mergeable 0|1 Set 0/1 to disable/enable the mergeable Rx feature. It’s disabled by default.

–stats interval The stats parameter controls the printing of virtio-net device statistics. The parameter specifies an interval (in unit of seconds) to print statistics, with an interval of 0 seconds disabling statistics.

–rx-retry 0|1 The rx-retry option enables/disables enqueue retries when the guests Rx queue is full. This feature resolves a packet loss that is observed at high data rates, by allowing it to delay and retry in the receive path. This option is enabled by default.

–rx-retry-num num The rx-retry-num option specifies the number of retries on an Rx burst, it takes effect only when rx retry is enabled. The default value is 4.

–rx-retry-delay msec The rx-retry-delay option specifies the timeout (in micro seconds) between retries on an RX burst, it takes effect only when rx retry is enabled. The default value is 15.

–dequeue-zero-copy Dequeue zero copy will be enabled when this option is given.

–vlan-strip 0|1 VLAN strip option is removed, because different NICs have different behaviors when disabling VLAN strip. Such feature, which heavily depends on hardware, should be removed from this example to reduce confusion. Now, VLAN strip is enabled and cannot be disabled.

35.4. Common Issues

  • QEMU fails to allocate memory on hugetlbfs, with an error like the following:

    file_ram_alloc: can't mmap RAM pages: Cannot allocate memory

    When running QEMU the above error indicates that it has failed to allocate memory for the Virtual Machine on the hugetlbfs. This is typically due to insufficient hugepages being free to support the allocation request. The number of free hugepages can be checked as follows:

    cat /sys/kernel/mm/hugepages/hugepages-<pagesize>/nr_hugepages

    The command above indicates how many hugepages are free to support QEMU’s allocation request.

  • vhost-user will not work with QEMU without the -mem-prealloc option

    The current implementation works properly only when the guest memory is pre-allocated.

  • vhost-user will not work with a QEMU version without shared memory mapping:

    Make sure share=on QEMU option is given.

  • Failed to build DPDK in VM

    Make sure “-cpu host” QEMU option is given.