26. 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 to tell the backend all the information it needs to know how to manipulate the vring.

Currently, there are two ways to pass these messages and as a result there are two Vhost implementations in DPDK: vhost-cuse (where the character devices are in user space) and vhost-user.

Vhost-cuse creates a user space character device and hook to a function ioctl, so that all ioctl commands that are sent from the frontend (QEMU) will be captured and handled.

Vhost-user creates a Unix domain socket file through which messages are passed.


Since DPDK v2.2, the majority of the development effort has gone into enhancing vhost-user, such as multiple queue, live migration, and reconnect. Thus, it is strongly advised to use vhost-user instead of vhost-cuse.

26.1. 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. For vhost-cuse, a /dev/path character device file will be created. For vhost-user server mode, a Unix domain socket file path will be created.

    Currently two flags are supported (these are valid for vhost-user only):


      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.

  • 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.

26.2. Vhost Implementations

26.2.1. Vhost-cuse implementation

When vSwitch registers the vhost driver, it will register a cuse device driver into the system and creates a character device file. This cuse driver will receive vhost open/release/IOCTL messages from the QEMU simulator.

When the open call is received, the vhost driver will create a vhost device for the virtio device in the guest.

When the VHOST_SET_MEM_TABLE ioctl is received, vhost searches the memory region to find the starting user space virtual address that maps the memory of the guest virtual machine. Through this virtual address and the QEMU pid, vhost can find the file QEMU uses to map the guest memory. Vhost maps this file into its address space, in this way vhost can fully access the guest physical memory, which means vhost could access the shared virtio ring and the guest physical address specified in the entry of the ring.

The guest virtual machine tells the vhost whether the virtio device is ready for processing or is de-activated through the VHOST_NET_SET_BACKEND message. The registered callback from vSwitch will be called.

When the release call is made, vhost will destroy the device.

26.2.2. Vhost-user implementation

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.

There is no VHOST_NET_SET_BACKEND message as in vhost-cuse to signal whether the virtio device is ready or stopped. Instead, 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.

26.3. 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.