8. Virtio_user as Exception Path


This solution is only applicable to Linux systems.

The virtual device, virtio-user, was originally introduced with the vhost-user backend as a high performance solution for IPC (Inter-Process Communication) and user space container networking.

Beyond this originally intended use, virtio-user can be used in conjunction with the vhost-kernel backend as a solution for dealing with exception path packets which need to be injected into the Linux kernel for processing there. In this regard, virtio-user and vhost in kernel space are an alternative to DPDK KNI for transferring packets between a DPDK packet processing application and the kernel stack.

This solution has a number of advantages over alternatives such as KNI:

  • Maintenance

    All kernel modules needed by this solution, vhost and vhost-net (kernel), are upstreamed and extensively used.

  • Features

    vhost-net is designed to be a networking solution, and, as such, has lots of networking related features, such as multi queue support, TSO, multi-segment buffer support, etc.

  • Performance

    Similar to KNI, this solution would use one or more kthreads to send/receive packets to/from user space DPDK applications, which minimises the impact on the polling DPDK threads.

The overview of an application using virtio-user as exception path is shown in Fig. 8.15.


Fig. 8.15 Overview of a DPDK app using virtio-user as exception path

8.1. Example Usage With Testpmd


These instructions assume that the vhost/vhost-net kernel modules are available and have already been loaded into the running kernel. It also assumes that the DPDK virtio driver has not been disabled in the DPDK build.

To run a simple test of virtio-user as exception path using testpmd:

  1. Compile DPDK and bind a NIC to vfio-pci as documented in Linux Drivers.

    This physical NIC is for communicating with the outside world, and serves as a packet source in this example.

  2. Run testpmd to forward packets from NIC to kernel, passing in a suitable list of logical cores to run on (-l parameter), and optionally the PCI address of the physical NIC to use (-a parameter). The virtio-user device for interfacing to the kernel is specified via a --vdev argument, taking the parameters described below.

    /path/to/dpdk-testpmd -l <cores> -a <pci BDF> \

    The path to the kernel vhost-net device.


    256 by default. To avoid shortage of descriptors, we can increase it to 1024.


    Number of virt-queues. Each queue will be served by a kthread.

  3. Once testpmd is running, a new network interface - called tap0 by default - will be present on the system. This should be configured with an IP address and then enabled for use:

    ip addr add dev tap0
    ip link set dev tap0 up
  4. To observe packet forwarding through the kernel, a second testpmd instance can be run on the system, taking packets from the kernel using an af_packet socket on the tap0 interface.

    /path/to/dpdk-testpmd -l <cores> --vdev=net_af_packet0,iface=tap0 --in-memory --no-pci

    When running this instance, we can use --in-memory flag to avoid hugepage naming conflicts with the previous instance, and we also use --no-pci flag to only use the af_packet interface for all traffic forwarding.

  5. Running traffic into the system through the NIC should see that traffic returned back again, having been forwarded through both testpmd instances. This can be confirmed by checking the testpmd statistics on testpmd exit.

For more advanced use of virtio-user with testpmd in this scenario, some other more advanced options may also be used. For example:

  • --tx-offloads=0x02c

    This testpmd option enables Tx offloads for UDP and TCP checksum on transmit, as well as TCP TSO support. The list of the offload flag values can be seen in header rte_ethdev.h.

  • --enable-lro

    This testpmd option is used to negotiate VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6 feature so that large packets from the kernel can be transmitted to the DPDK application and further TSOed by physical NIC. If unsupported by the physical NIC, errors may be reported by testpmd with this option.

  • Enabling Rx checksum offloads for physical port:

    Within testpmd, you can enable and disable offloads on a per-port basis, rather than enabling them for both ports. For the physical NIC, it may be desirable to enable checksum offload on packet Rx. This may be done as below, if testpmd is run with -i flag for interactive mode.

    testpmd> port stop 0
    testpmd> port config 0 rx_offload tcp_cksum on
    testpmd> port config 0 rx_offload udp_cksum on
    testpmd> port start 0
  • Multiple queue support

    Better performance may be achieved by using multiple queues, so that multiple kernel threads are handling the traffic on the kernel side. For example, to use 2 queues on both NIC and virtio ports, while also enabling TX offloads and LRO support:

    /path/to/dpdk-testpmd --vdev=virtio_user0,path=/dev/vhost-net,queues=2,queue_size=1024 -- \
        -i --tx-offloads=0x002c --enable-lro --txq=2 --rxq=2 --txd=1024 --rxd=1024

8.2. Creating Virtio-User Ports within an Application

To use virtio-user ports within an application, it is not necessary to explicitly initialize those ports using EAL arguments at startup. Instead, one can use the generic EAL API rte_eal_hotplug_add function to create a new instance at startup. For example, to create a basic virtio-user port, the following code could be used:

rte_eal_hotplug_add("vdev", "virtio_user0", "path=/dev/vhost-net");

A fuller code example is shown below, where a virtio-user port, and hence kernel netdev, is created for each NIC port discovered by DPDK. Each virtio-user port is given the MAC address of its matching physical port (assuming app was run without vdev args on command line, so all ports auto-discovered are HW ones). These new virtio-user netdevs will appear in the kernel port listings as virtio_user0, virtio_user1, etc., based on the names passed in as iface= via the portargs parameter.

nb_ports = rte_eth_dev_count_avail();

/* Create a vhost_user port for each physical port */
unsigned port_count = 0;
    char portname[32];
    char portargs[256];
    struct rte_ether_addr addr = {0};

    /* once we have created a virtio port for each physical port, stop creating more */
    if (++port_count > nb_ports)

    /* get MAC address of physical port to use as MAC of virtio_user port */
    rte_eth_macaddr_get(portid, &addr);

    /* set the name and arguments */
    snprintf(portname, sizeof(portname), "virtio_user%u", portid);
    snprintf(portargs, sizeof(portargs),
            "path=/dev/vhost-net,queues=1,queue_size=%u,iface=%s,mac=" RTE_ETHER_ADDR_PRT_FMT,
            RX_RING_SIZE, portname, RTE_ETHER_ADDR_BYTES(&addr));

    /* add the vdev for virtio_user */
    if (rte_eal_hotplug_add("vdev", portname, portargs) < 0)
        rte_exit(EXIT_FAILURE, "Cannot create paired port for port %u\n", portid);


Once these virtio-user ports have been created in the loop, all ports, both physical and virtual, may be initialized and used as normal in the application.