27. Vhost Library

The vhost library implements a user space virtio net server allowing the user to manipulate the virtio ring directly. In another words, it allows the user to fetch/put packets from/to the VM virtio net device. To achieve this, a vhost library should be able to:

  • Access the guest memory:

    For QEMU, this is done by using the -object memory-backend-file,share=on,... option. Which means QEMU will create a file to serve as the guest RAM. The share=on option allows another process to map that file, which means it can access the guest RAM.

  • Know all the necessary information about the vring:

    Information such as where the available ring is stored. Vhost defines some messages (passed through a Unix domain socket file) to tell the backend all the information it needs to know how to manipulate the vring.

27.1. Vhost API Overview

The following is an overview of the Vhost API functions:

  • rte_vhost_driver_register(path, flags)

    This function registers a vhost driver into the system. path specifies the Unix domain socket file path.

    Currently supported flags are:


      DPDK vhost-user will act as the client when this flag is given. See below for an explanation.


      When DPDK vhost-user acts as the client it will keep trying to reconnect to the server (QEMU) until it succeeds. This is useful in two cases:

      • When QEMU is not started yet.
      • When QEMU restarts (for example due to a guest OS reboot).

      This reconnect option is enabled by default. However, it can be turned off by setting this flag.


      Dequeue zero copy will be enabled when this flag is set. It is disabled by default.

      There are some truths (including limitations) you might want to know while setting this flag:

      • zero copy is not good for small packets (typically for packet size below 512).

      • zero copy is really good for VM2VM case. For iperf between two VMs, the boost could be above 70% (when TSO is enableld).

      • for VM2NIC case, the nb_tx_desc has to be small enough: <= 64 if virtio indirect feature is not enabled and <= 128 if it is enabled.

        This is because when dequeue zero copy is enabled, guest Tx used vring will be updated only when corresponding mbuf is freed. Thus, the nb_tx_desc has to be small enough so that the PMD driver will run out of available Tx descriptors and free mbufs timely. Otherwise, guest Tx vring would be starved.

      • Guest memory should be backended with huge pages to achieve better performance. Using 1G page size is the best.

        When dequeue zero copy is enabled, the guest phys address and host phys address mapping has to be established. Using non-huge pages means far more page segments. To make it simple, DPDK vhost does a linear search of those segments, thus the fewer the segments, the quicker we will get the mapping. NOTE: we may speed it by using tree searching in future.

  • rte_vhost_driver_session_start()

    This function starts the vhost session loop to handle vhost messages. It starts an infinite loop, therefore it should be called in a dedicated thread.

  • rte_vhost_driver_callback_register(virtio_net_device_ops)

    This function registers a set of callbacks, to let DPDK applications take the appropriate action when some events happen. The following events are currently supported:

    • new_device(int vid)

      This callback is invoked when a virtio net device becomes ready. vid is the virtio net device ID.

    • destroy_device(int vid)

      This callback is invoked when a virtio net device shuts down (or when the vhost connection is broken).

    • vring_state_changed(int vid, uint16_t queue_id, int enable)

      This callback is invoked when a specific queue’s state is changed, for example to enabled or disabled.

  • rte_vhost_enqueue_burst(vid, queue_id, pkts, count)

    Transmits (enqueues) count packets from host to guest.

  • rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)

    Receives (dequeues) count packets from guest, and stored them at pkts.

  • rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)

    This function disables/enables some features. For example, it can be used to disable mergeable buffers and TSO features, which both are enabled by default.

27.2. Vhost-user Implementations

Vhost-user uses Unix domain sockets for passing messages. This means the DPDK vhost-user implementation has two options:

  • DPDK vhost-user acts as the server.

    DPDK will create a Unix domain socket server file and listen for connections from the frontend.

    Note, this is the default mode, and the only mode before DPDK v16.07.

  • DPDK vhost-user acts as the client.

    Unlike the server mode, this mode doesn’t create the socket file; it just tries to connect to the server (which responses to create the file instead).

    When the DPDK vhost-user application restarts, DPDK vhost-user will try to connect to the server again. This is how the “reconnect” feature works.


    • The “reconnect” feature requires QEMU v2.7 (or above).
    • The vhost supported features must be exactly the same before and after the restart. For example, if TSO is disabled and then enabled, nothing will work and issues undefined might happen.

No matter which mode is used, once a connection is established, DPDK vhost-user will start receiving and processing vhost messages from QEMU.

For messages with a file descriptor, the file descriptor can be used directly in the vhost process as it is already installed by the Unix domain socket.

The supported vhost messages are:


For VHOST_SET_MEM_TABLE message, QEMU will send information for each memory region and its file descriptor in the ancillary data of the message. The file descriptor is used to map that region.

VHOST_SET_VRING_KICK is used as the signal to put the vhost device into the data plane, and VHOST_GET_VRING_BASE is used as the signal to remove the vhost device from the data plane.

When the socket connection is closed, vhost will destroy the device.

27.3. Vhost supported vSwitch reference

For more vhost details and how to support vhost in vSwitch, please refer to the vhost example in the DPDK Sample Applications Guide.