1.. SPDX-License-Identifier: BSD-3-Clause 2 Copyright(c) 2010-2016 Intel Corporation. 3 4Vhost Library 5============= 6 7The vhost library implements a user space virtio net server allowing the user 8to manipulate the virtio ring directly. In another words, it allows the user 9to fetch/put packets from/to the VM virtio net device. To achieve this, a 10vhost library should be able to: 11 12* Access the guest memory: 13 14 For QEMU, this is done by using the ``-object memory-backend-file,share=on,...`` 15 option. Which means QEMU will create a file to serve as the guest RAM. 16 The ``share=on`` option allows another process to map that file, which 17 means it can access the guest RAM. 18 19* Know all the necessary information about the vring: 20 21 Information such as where the available ring is stored. Vhost defines some 22 messages (passed through a Unix domain socket file) to tell the backend all 23 the information it needs to know how to manipulate the vring. 24 25 26Vhost API Overview 27------------------ 28 29The following is an overview of some key Vhost API functions: 30 31* ``rte_vhost_driver_register(path, flags)`` 32 33 This function registers a vhost driver into the system. ``path`` specifies 34 the Unix domain socket file path. 35 36 Currently supported flags are: 37 38 - ``RTE_VHOST_USER_CLIENT`` 39 40 DPDK vhost-user will act as the client when this flag is given. See below 41 for an explanation. 42 43 - ``RTE_VHOST_USER_NO_RECONNECT`` 44 45 When DPDK vhost-user acts as the client it will keep trying to reconnect 46 to the server (QEMU) until it succeeds. This is useful in two cases: 47 48 * When QEMU is not started yet. 49 * When QEMU restarts (for example due to a guest OS reboot). 50 51 This reconnect option is enabled by default. However, it can be turned off 52 by setting this flag. 53 54 - ``RTE_VHOST_USER_DEQUEUE_ZERO_COPY`` 55 56 Dequeue zero copy will be enabled when this flag is set. It is disabled by 57 default. 58 59 There are some truths (including limitations) you might want to know while 60 setting this flag: 61 62 * zero copy is not good for small packets (typically for packet size below 63 512). 64 65 * zero copy is really good for VM2VM case. For iperf between two VMs, the 66 boost could be above 70% (when TSO is enableld). 67 68 * for VM2NIC case, the ``nb_tx_desc`` has to be small enough: <= 64 if virtio 69 indirect feature is not enabled and <= 128 if it is enabled. 70 71 This is because when dequeue zero copy is enabled, guest Tx used vring will 72 be updated only when corresponding mbuf is freed. Thus, the nb_tx_desc 73 has to be small enough so that the PMD driver will run out of available 74 Tx descriptors and free mbufs timely. Otherwise, guest Tx vring would be 75 starved. 76 77 * Guest memory should be backended with huge pages to achieve better 78 performance. Using 1G page size is the best. 79 80 When dequeue zero copy is enabled, the guest phys address and host phys 81 address mapping has to be established. Using non-huge pages means far 82 more page segments. To make it simple, DPDK vhost does a linear search 83 of those segments, thus the fewer the segments, the quicker we will get 84 the mapping. NOTE: we may speed it by using tree searching in future. 85 86 * zero copy can not work when using vfio-pci with iommu mode currently, this 87 is because we don't setup iommu dma mapping for guest memory. If you have 88 to use vfio-pci driver, please insert vfio-pci kernel module in noiommu 89 mode. 90 91 - ``RTE_VHOST_USER_IOMMU_SUPPORT`` 92 93 IOMMU support will be enabled when this flag is set. It is disabled by 94 default. 95 96 Enabling this flag makes possible to use guest vIOMMU to protect vhost 97 from accessing memory the virtio device isn't allowed to, when the feature 98 is negotiated and an IOMMU device is declared. 99 100 However, this feature enables vhost-user's reply-ack protocol feature, 101 which implementation is buggy in Qemu v2.7.0-v2.9.0 when doing multiqueue. 102 Enabling this flag with these Qemu version results in Qemu being blocked 103 when multiple queue pairs are declared. 104 105* ``rte_vhost_driver_set_features(path, features)`` 106 107 This function sets the feature bits the vhost-user driver supports. The 108 vhost-user driver could be vhost-user net, yet it could be something else, 109 say, vhost-user SCSI. 110 111* ``rte_vhost_driver_callback_register(path, vhost_device_ops)`` 112 113 This function registers a set of callbacks, to let DPDK applications take 114 the appropriate action when some events happen. The following events are 115 currently supported: 116 117 * ``new_device(int vid)`` 118 119 This callback is invoked when a virtio device becomes ready. ``vid`` 120 is the vhost device ID. 121 122 * ``destroy_device(int vid)`` 123 124 This callback is invoked when a virtio device is paused or shut down. 125 126 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)`` 127 128 This callback is invoked when a specific queue's state is changed, for 129 example to enabled or disabled. 130 131 * ``features_changed(int vid, uint64_t features)`` 132 133 This callback is invoked when the features is changed. For example, 134 ``VHOST_F_LOG_ALL`` will be set/cleared at the start/end of live 135 migration, respectively. 136 137 * ``new_connection(int vid)`` 138 139 This callback is invoked on new vhost-user socket connection. If DPDK 140 acts as the server the device should not be deleted before 141 ``destroy_connection`` callback is received. 142 143 * ``destroy_connection(int vid)`` 144 145 This callback is invoked when vhost-user socket connection is closed. 146 It indicates that device with id ``vid`` is no longer in use and can be 147 safely deleted. 148 149* ``rte_vhost_driver_disable/enable_features(path, features))`` 150 151 This function disables/enables some features. For example, it can be used to 152 disable mergeable buffers and TSO features, which both are enabled by 153 default. 154 155* ``rte_vhost_driver_start(path)`` 156 157 This function triggers the vhost-user negotiation. It should be invoked at 158 the end of initializing a vhost-user driver. 159 160* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)`` 161 162 Transmits (enqueues) ``count`` packets from host to guest. 163 164* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)`` 165 166 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``. 167 168* ``rte_vhost_crypto_create(vid, cryptodev_id, sess_mempool, socket_id)`` 169 170 As an extension of new_device(), this function adds virtio-crypto workload 171 acceleration capability to the device. All crypto workload is processed by 172 DPDK cryptodev with the device ID of ``cryptodev_id``. 173 174* ``rte_vhost_crypto_free(vid)`` 175 176 Frees the memory and vhost-user message handlers created in 177 rte_vhost_crypto_create(). 178 179* ``rte_vhost_crypto_fetch_requests(vid, queue_id, ops, nb_ops)`` 180 181 Receives (dequeues) ``nb_ops`` virtio-crypto requests from guest, parses 182 them to DPDK Crypto Operations, and fills the ``ops`` with parsing results. 183 184* ``rte_vhost_crypto_finalize_requests(queue_id, ops, nb_ops)`` 185 186 After the ``ops`` are dequeued from Cryptodev, finalizes the jobs and 187 notifies the guest(s). 188 189* ``rte_vhost_crypto_set_zero_copy(vid, option)`` 190 191 Enable or disable zero copy feature of the vhost crypto backend. 192 193Vhost-user Implementations 194-------------------------- 195 196Vhost-user uses Unix domain sockets for passing messages. This means the DPDK 197vhost-user implementation has two options: 198 199* DPDK vhost-user acts as the server. 200 201 DPDK will create a Unix domain socket server file and listen for 202 connections from the frontend. 203 204 Note, this is the default mode, and the only mode before DPDK v16.07. 205 206 207* DPDK vhost-user acts as the client. 208 209 Unlike the server mode, this mode doesn't create the socket file; 210 it just tries to connect to the server (which responses to create the 211 file instead). 212 213 When the DPDK vhost-user application restarts, DPDK vhost-user will try to 214 connect to the server again. This is how the "reconnect" feature works. 215 216 .. Note:: 217 * The "reconnect" feature requires **QEMU v2.7** (or above). 218 219 * The vhost supported features must be exactly the same before and 220 after the restart. For example, if TSO is disabled and then enabled, 221 nothing will work and issues undefined might happen. 222 223No matter which mode is used, once a connection is established, DPDK 224vhost-user will start receiving and processing vhost messages from QEMU. 225 226For messages with a file descriptor, the file descriptor can be used directly 227in the vhost process as it is already installed by the Unix domain socket. 228 229The supported vhost messages are: 230 231* ``VHOST_SET_MEM_TABLE`` 232* ``VHOST_SET_VRING_KICK`` 233* ``VHOST_SET_VRING_CALL`` 234* ``VHOST_SET_LOG_FD`` 235* ``VHOST_SET_VRING_ERR`` 236 237For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each 238memory region and its file descriptor in the ancillary data of the message. 239The file descriptor is used to map that region. 240 241``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into 242the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove 243the vhost device from the data plane. 244 245When the socket connection is closed, vhost will destroy the device. 246 247Guest memory requirement 248------------------------ 249 250* Memory pre-allocation 251 252 For non-zerocopy, guest memory pre-allocation is not a must. This can help 253 save of memory. If users really want the guest memory to be pre-allocated 254 (e.g., for performance reason), we can add option ``-mem-prealloc`` when 255 starting QEMU. Or, we can lock all memory at vhost side which will force 256 memory to be allocated when mmap at vhost side; option --mlockall in 257 ovs-dpdk is an example in hand. 258 259 For zerocopy, we force the VM memory to be pre-allocated at vhost lib when 260 mapping the guest memory; and also we need to lock the memory to prevent 261 pages being swapped out to disk. 262 263* Memory sharing 264 265 Make sure ``share=on`` QEMU option is given. vhost-user will not work with 266 a QEMU version without shared memory mapping. 267 268Vhost supported vSwitch reference 269--------------------------------- 270 271For more vhost details and how to support vhost in vSwitch, please refer to 272the vhost example in the DPDK Sample Applications Guide. 273