33. 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 Intel® 82599 10 Gigabit Ethernet Controller.

33.1. Testing steps

This section shows the steps how to test a typical PVP case with this dpdk-vhost 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 dpdk-vhost example, in turn, gets the packets and puts back to the same physical NIC port.

33.1.1. 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.

. _vhost_app_run_vm:

33.1.2. 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.

33.1.3. Start the vswitch example

./dpdk-vhost -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.

33.1.4. 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 vfio-pci
dpdk/usertools/dpdk-devbind.py -b vfio-pci 0000:00:04.0

Then start testpmd for packet forwarding testing.

./<build_dir>/app/dpdk-testpmd -l 0-1 -- -i
> start tx_first

For more information about vIOMMU and NO-IOMMU and VFIO please refer to Linux Drivers section of the DPDK Getting started guide.

33.2. Inject packets

While a virtio-net is connected to dpdk-vhost, 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 dpdk-vhost console. It means you get it work:

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

33.3. 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, implying 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.

–builtin-net-driver A very simple vhost-user net driver which demonstrates how to use the generic vhost APIs will be used when this option is given. It is disabled by default.

–dmas This parameter is used to specify the assigned DMA device of a vhost device. Async vhost-user net driver will be used if –dmas is set. For example –dmas [txd0@00:04.0,txd1@00:04.1,rxd0@00:04.2,rxd1@00:04.3] means use DMA channel 00:04.0/00:04.2 for vhost device 0 enqueue/dequeue operation and use DMA channel 00:04.1/00:04.3 for vhost device 1 enqueue/dequeue operation. The index of the device corresponds to the socket file in order, that means vhost device 0 is created through the first socket file, vhost device 1 is created through the second socket file, and so on.

–total-num-mbufs 0-N This parameter sets the number of mbufs to be allocated in mbuf pools, the default value is 147456. This is can be used if launch of a port fails due to shortage of mbufs.

–tso 0|1 Disables/enables TCP segment offload.

–tx-csum 0|1 Disables/enables TX checksum offload.

-p mask Port mask which specifies the ports to be used

33.4. 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:

    dpdk-hugepages.py --show

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

  • Failed to build DPDK in VM

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

  • Device start fails if NIC’s max queues > the default number of 128

    mbuf pool size is dependent on the MAX_QUEUES configuration, if NIC’s max queue number is larger than 128, device start will fail due to insufficient mbuf. This can be adjusted using --total-num-mbufs parameter.

  • Option “builtin-net-driver” is incompatible with QEMU

    QEMU vhost net device start will fail if protocol feature is not negotiated. DPDK virtio-user PMD can be the replacement of QEMU.

  • Device start fails when enabling “builtin-net-driver” without memory pre-allocation

    The builtin example doesn’t support dynamic memory allocation. When vhost backend enables “builtin-net-driver”, “–socket-mem” option should be added at virtio-user PMD side as a startup item.