24. Vhost Library

The vhost library implements a user space vhost driver. It supports both vhost-cuse (cuse: user space character device) and vhost-user(user space socket server). It also creates, manages and destroys vhost devices for corresponding virtio devices in the guest. Vhost supported vSwitch could register callbacks to this library, which will be called when a vhost device is activated or deactivated by guest virtual machine.

24.1. Vhost API Overview

  • Vhost driver registration

    rte_vhost_driver_register registers the vhost driver into the system. For vhost-cuse, character device file will be created under the /dev directory. Character device name is specified as the parameter. For vhost-user, a Unix domain socket server will be created with the parameter as the local socket path.

  • Vhost session start

    rte_vhost_driver_session_start starts the vhost session loop. Vhost session is an infinite blocking loop. Put the session in a dedicate DPDK thread.

  • Callback register

    Vhost supported vSwitch could call rte_vhost_driver_callback_register to register two callbacks, new_destory and destroy_device. When virtio device is activated or deactivated by guest virtual machine, the callback will be called, then vSwitch could put the device onto data core or remove the device from data core by setting or unsetting VIRTIO_DEV_RUNNING on the device flags.

  • Read/write packets from/to guest virtual machine

    rte_vhost_enqueue_burst transmit host packets to guest. rte_vhost_dequeue_burst receives packets from guest.

  • Feature enable/disable

    Now one negotiate-able feature in vhost is merge-able. vSwitch could enable/disable this feature for performance consideration.

24.2. Vhost Implementation

24.2.1. 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 message from QEMU simulator.

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

When 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 guest virtual machine. Through this virtual address and the QEMU pid, vhost could find the file QEMU uses to map the guest memory. Vhost maps this file into its address space, in this way vhost could 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 VHOST_NET_SET_BACKEND message. The registered callback from vSwitch will be called.

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

24.2.2. Vhost user implementation

When vSwitch registers a vhost driver, it will create a Unix domain socket server into the system. This server will listen for a connection and process the vhost message from QEMU simulator.

When there is a new socket connection, it means a new virtio device has been created in the guest virtual machine, and the vhost driver will create a vhost device for this virtio device.

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


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

There is no VHOST_NET_SET_BACKEND message as in vhost cuse to signal us whether virtio device is ready or should be stopped. VHOST_SET_VRING_KICK is used as the signal to put the vhost device onto data plane. VHOST_GET_VRING_BASE is used as the signal to remove vhost device from data plane.

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

24.3. Vhost supported vSwitch reference

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