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.. BSD LICENSE
Copyright(c) 2010-2014 Intel Corporation. All rights reserved.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in
the documentation and/or other materials provided with the
distribution.
* Neither the name of Intel Corporation nor the names of its
contributors may be used to endorse or promote products derived
from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.
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.
Vhost Implementation
--------------------
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.
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.
* VHOST_SET_MEM_TABLE
* VHOST_SET_VRING_KICK
* VHOST_SET_VRING_CALL
* VHOST_SET_LOG_FD
* VHOST_SET_VRING_ERR
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.
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.
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