1.. BSD LICENSE 2 Copyright(c) 2010-2016 Intel Corporation. All rights reserved. 3 All rights reserved. 4 5 Redistribution and use in source and binary forms, with or without 6 modification, are permitted provided that the following conditions 7 are met: 8 9 * Redistributions of source code must retain the above copyright 10 notice, this list of conditions and the following disclaimer. 11 * Redistributions in binary form must reproduce the above copyright 12 notice, this list of conditions and the following disclaimer in 13 the documentation and/or other materials provided with the 14 distribution. 15 * Neither the name of Intel Corporation nor the names of its 16 contributors may be used to endorse or promote products derived 17 from this software without specific prior written permission. 18 19 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 20 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 21 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 22 A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 23 OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 24 SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 25 LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 26 DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 27 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 28 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 29 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 30 31Vhost Library 32============= 33 34The vhost library implements a user space virtio net server allowing the user 35to manipulate the virtio ring directly. In another words, it allows the user 36to fetch/put packets from/to the VM virtio net device. To achieve this, a 37vhost library should be able to: 38 39* Access the guest memory: 40 41 For QEMU, this is done by using the ``-object memory-backend-file,share=on,...`` 42 option. Which means QEMU will create a file to serve as the guest RAM. 43 The ``share=on`` option allows another process to map that file, which 44 means it can access the guest RAM. 45 46* Know all the necessary information about the vring: 47 48 Information such as where the available ring is stored. Vhost defines some 49 messages to tell the backend all the information it needs to know how to 50 manipulate the vring. 51 52Currently, there are two ways to pass these messages and as a result there are 53two Vhost implementations in DPDK: *vhost-cuse* (where the character devices 54are in user space) and *vhost-user*. 55 56Vhost-cuse creates a user space character device and hook to a function ioctl, 57so that all ioctl commands that are sent from the frontend (QEMU) will be 58captured and handled. 59 60Vhost-user creates a Unix domain socket file through which messages are 61passed. 62 63.. Note:: 64 65 Since DPDK v2.2, the majority of the development effort has gone into 66 enhancing vhost-user, such as multiple queue, live migration, and 67 reconnect. Thus, it is strongly advised to use vhost-user instead of 68 vhost-cuse. 69 70 71Vhost API Overview 72------------------ 73 74The following is an overview of the Vhost API functions: 75 76* ``rte_vhost_driver_register(path, flags)`` 77 78 This function registers a vhost driver into the system. For vhost-cuse, a 79 ``/dev/path`` character device file will be created. For vhost-user server 80 mode, a Unix domain socket file ``path`` will be created. 81 82 Currently two flags are supported (these are valid for vhost-user only): 83 84 - ``RTE_VHOST_USER_CLIENT`` 85 86 DPDK vhost-user will act as the client when this flag is given. See below 87 for an explanation. 88 89 - ``RTE_VHOST_USER_NO_RECONNECT`` 90 91 When DPDK vhost-user acts as the client it will keep trying to reconnect 92 to the server (QEMU) until it succeeds. This is useful in two cases: 93 94 * When QEMU is not started yet. 95 * When QEMU restarts (for example due to a guest OS reboot). 96 97 This reconnect option is enabled by default. However, it can be turned off 98 by setting this flag. 99 100* ``rte_vhost_driver_session_start()`` 101 102 This function starts the vhost session loop to handle vhost messages. It 103 starts an infinite loop, therefore it should be called in a dedicated 104 thread. 105 106* ``rte_vhost_driver_callback_register(virtio_net_device_ops)`` 107 108 This function registers a set of callbacks, to let DPDK applications take 109 the appropriate action when some events happen. The following events are 110 currently supported: 111 112 * ``new_device(int vid)`` 113 114 This callback is invoked when a virtio net device becomes ready. ``vid`` 115 is the virtio net device ID. 116 117 * ``destroy_device(int vid)`` 118 119 This callback is invoked when a virtio net device shuts down (or when the 120 vhost connection is broken). 121 122 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)`` 123 124 This callback is invoked when a specific queue's state is changed, for 125 example to enabled or disabled. 126 127* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)`` 128 129 Transmits (enqueues) ``count`` packets from host to guest. 130 131* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)`` 132 133 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``. 134 135* ``rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)`` 136 137 This function disables/enables some features. For example, it can be used to 138 disable mergeable buffers and TSO features, which both are enabled by 139 default. 140 141 142Vhost Implementations 143--------------------- 144 145Vhost-cuse implementation 146~~~~~~~~~~~~~~~~~~~~~~~~~ 147 148When vSwitch registers the vhost driver, it will register a cuse device driver 149into the system and creates a character device file. This cuse driver will 150receive vhost open/release/IOCTL messages from the QEMU simulator. 151 152When the open call is received, the vhost driver will create a vhost device 153for the virtio device in the guest. 154 155When the ``VHOST_SET_MEM_TABLE`` ioctl is received, vhost searches the memory 156region to find the starting user space virtual address that maps the memory of 157the guest virtual machine. Through this virtual address and the QEMU pid, 158vhost can find the file QEMU uses to map the guest memory. Vhost maps this 159file into its address space, in this way vhost can fully access the guest 160physical memory, which means vhost could access the shared virtio ring and the 161guest physical address specified in the entry of the ring. 162 163The guest virtual machine tells the vhost whether the virtio device is ready 164for processing or is de-activated through the ``VHOST_NET_SET_BACKEND`` 165message. The registered callback from vSwitch will be called. 166 167When the release call is made, vhost will destroy the device. 168 169Vhost-user implementation 170~~~~~~~~~~~~~~~~~~~~~~~~~ 171 172Vhost-user uses Unix domain sockets for passing messages. This means the DPDK 173vhost-user implementation has two options: 174 175* DPDK vhost-user acts as the server. 176 177 DPDK will create a Unix domain socket server file and listen for 178 connections from the frontend. 179 180 Note, this is the default mode, and the only mode before DPDK v16.07. 181 182 183* DPDK vhost-user acts as the client. 184 185 Unlike the server mode, this mode doesn't create the socket file; 186 it just tries to connect to the server (which responses to create the 187 file instead). 188 189 When the DPDK vhost-user application restarts, DPDK vhost-user will try to 190 connect to the server again. This is how the "reconnect" feature works. 191 192 Note: the "reconnect" feature requires **QEMU v2.7** (or above). 193 194No matter which mode is used, once a connection is established, DPDK 195vhost-user will start receiving and processing vhost messages from QEMU. 196 197For messages with a file descriptor, the file descriptor can be used directly 198in the vhost process as it is already installed by the Unix domain socket. 199 200The supported vhost messages are: 201 202* ``VHOST_SET_MEM_TABLE`` 203* ``VHOST_SET_VRING_KICK`` 204* ``VHOST_SET_VRING_CALL`` 205* ``VHOST_SET_LOG_FD`` 206* ``VHOST_SET_VRING_ERR`` 207 208For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each 209memory region and its file descriptor in the ancillary data of the message. 210The file descriptor is used to map that region. 211 212There is no ``VHOST_NET_SET_BACKEND`` message as in vhost-cuse to signal 213whether the virtio device is ready or stopped. Instead, 214``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into 215the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove 216the vhost device from the data plane. 217 218When the socket connection is closed, vhost will destroy the device. 219 220Vhost supported vSwitch reference 221--------------------------------- 222 223For more vhost details and how to support vhost in vSwitch, please refer to 224the vhost example in the DPDK Sample Applications Guide. 225