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 (passed through a Unix domain socket file) to tell the backend all 50 the information it needs to know how to manipulate the vring. 51 52 53Vhost API Overview 54------------------ 55 56The following is an overview of some key Vhost API functions: 57 58* ``rte_vhost_driver_register(path, flags)`` 59 60 This function registers a vhost driver into the system. ``path`` specifies 61 the Unix domain socket file path. 62 63 Currently supported flags are: 64 65 - ``RTE_VHOST_USER_CLIENT`` 66 67 DPDK vhost-user will act as the client when this flag is given. See below 68 for an explanation. 69 70 - ``RTE_VHOST_USER_NO_RECONNECT`` 71 72 When DPDK vhost-user acts as the client it will keep trying to reconnect 73 to the server (QEMU) until it succeeds. This is useful in two cases: 74 75 * When QEMU is not started yet. 76 * When QEMU restarts (for example due to a guest OS reboot). 77 78 This reconnect option is enabled by default. However, it can be turned off 79 by setting this flag. 80 81 - ``RTE_VHOST_USER_DEQUEUE_ZERO_COPY`` 82 83 Dequeue zero copy will be enabled when this flag is set. It is disabled by 84 default. 85 86 There are some truths (including limitations) you might want to know while 87 setting this flag: 88 89 * zero copy is not good for small packets (typically for packet size below 90 512). 91 92 * zero copy is really good for VM2VM case. For iperf between two VMs, the 93 boost could be above 70% (when TSO is enableld). 94 95 * for VM2NIC case, the ``nb_tx_desc`` has to be small enough: <= 64 if virtio 96 indirect feature is not enabled and <= 128 if it is enabled. 97 98 This is because when dequeue zero copy is enabled, guest Tx used vring will 99 be updated only when corresponding mbuf is freed. Thus, the nb_tx_desc 100 has to be small enough so that the PMD driver will run out of available 101 Tx descriptors and free mbufs timely. Otherwise, guest Tx vring would be 102 starved. 103 104 * Guest memory should be backended with huge pages to achieve better 105 performance. Using 1G page size is the best. 106 107 When dequeue zero copy is enabled, the guest phys address and host phys 108 address mapping has to be established. Using non-huge pages means far 109 more page segments. To make it simple, DPDK vhost does a linear search 110 of those segments, thus the fewer the segments, the quicker we will get 111 the mapping. NOTE: we may speed it by using tree searching in future. 112 113 - ``RTE_VHOST_USER_IOMMU_SUPPORT`` 114 115 IOMMU support will be enabled when this flag is set. It is disabled by 116 default. 117 118 Enabling this flag makes possible to use guest vIOMMU to protect vhost 119 from accessing memory the virtio device isn't allowed to, when the feature 120 is negotiated and an IOMMU device is declared. 121 122 However, this feature enables vhost-user's reply-ack protocol feature, 123 which implementation is buggy in Qemu v2.7.0-v2.9.0 when doing multiqueue. 124 Enabling this flag with these Qemu version results in Qemu being blocked 125 when multiple queue pairs are declared. 126 127* ``rte_vhost_driver_set_features(path, features)`` 128 129 This function sets the feature bits the vhost-user driver supports. The 130 vhost-user driver could be vhost-user net, yet it could be something else, 131 say, vhost-user SCSI. 132 133* ``rte_vhost_driver_callback_register(path, vhost_device_ops)`` 134 135 This function registers a set of callbacks, to let DPDK applications take 136 the appropriate action when some events happen. The following events are 137 currently supported: 138 139 * ``new_device(int vid)`` 140 141 This callback is invoked when a virtio device becomes ready. ``vid`` 142 is the vhost device ID. 143 144 * ``destroy_device(int vid)`` 145 146 This callback is invoked when a virtio device is paused or shut down. 147 148 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)`` 149 150 This callback is invoked when a specific queue's state is changed, for 151 example to enabled or disabled. 152 153 * ``features_changed(int vid, uint64_t features)`` 154 155 This callback is invoked when the features is changed. For example, 156 ``VHOST_F_LOG_ALL`` will be set/cleared at the start/end of live 157 migration, respectively. 158 159 * ``new_connection(int vid)`` 160 161 This callback is invoked on new vhost-user socket connection. If DPDK 162 acts as the server the device should not be deleted before 163 ``destroy_connection`` callback is received. 164 165 * ``destroy_connection(int vid)`` 166 167 This callback is invoked when vhost-user socket connection is closed. 168 It indicates that device with id ``vid`` is no longer in use and can be 169 safely deleted. 170 171* ``rte_vhost_driver_disable/enable_features(path, features))`` 172 173 This function disables/enables some features. For example, it can be used to 174 disable mergeable buffers and TSO features, which both are enabled by 175 default. 176 177* ``rte_vhost_driver_start(path)`` 178 179 This function triggers the vhost-user negotiation. It should be invoked at 180 the end of initializing a vhost-user driver. 181 182* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)`` 183 184 Transmits (enqueues) ``count`` packets from host to guest. 185 186* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)`` 187 188 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``. 189 190Vhost-user Implementations 191-------------------------- 192 193Vhost-user uses Unix domain sockets for passing messages. This means the DPDK 194vhost-user implementation has two options: 195 196* DPDK vhost-user acts as the server. 197 198 DPDK will create a Unix domain socket server file and listen for 199 connections from the frontend. 200 201 Note, this is the default mode, and the only mode before DPDK v16.07. 202 203 204* DPDK vhost-user acts as the client. 205 206 Unlike the server mode, this mode doesn't create the socket file; 207 it just tries to connect to the server (which responses to create the 208 file instead). 209 210 When the DPDK vhost-user application restarts, DPDK vhost-user will try to 211 connect to the server again. This is how the "reconnect" feature works. 212 213 .. Note:: 214 * The "reconnect" feature requires **QEMU v2.7** (or above). 215 216 * The vhost supported features must be exactly the same before and 217 after the restart. For example, if TSO is disabled and then enabled, 218 nothing will work and issues undefined might happen. 219 220No matter which mode is used, once a connection is established, DPDK 221vhost-user will start receiving and processing vhost messages from QEMU. 222 223For messages with a file descriptor, the file descriptor can be used directly 224in the vhost process as it is already installed by the Unix domain socket. 225 226The supported vhost messages are: 227 228* ``VHOST_SET_MEM_TABLE`` 229* ``VHOST_SET_VRING_KICK`` 230* ``VHOST_SET_VRING_CALL`` 231* ``VHOST_SET_LOG_FD`` 232* ``VHOST_SET_VRING_ERR`` 233 234For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each 235memory region and its file descriptor in the ancillary data of the message. 236The file descriptor is used to map that region. 237 238``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into 239the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove 240the vhost device from the data plane. 241 242When the socket connection is closed, vhost will destroy the device. 243 244Vhost supported vSwitch reference 245--------------------------------- 246 247For more vhost details and how to support vhost in vSwitch, please refer to 248the vhost example in the DPDK Sample Applications Guide. 249