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 zero copy in VM2NIC case, guest Tx used vring may be starved if the 69 PMD driver consume the mbuf but not release them timely. 70 71 For example, i40e driver has an optimization to maximum NIC pipeline which 72 postpones returning transmitted mbuf until only tx_free_threshold free 73 descs left. The virtio TX used ring will be starved if the formula 74 (num_i40e_tx_desc - num_virtio_tx_desc > tx_free_threshold) is true, since 75 i40e will not return back mbuf. 76 77 A performance tip for tuning zero copy in VM2NIC case is to adjust the 78 frequency of mbuf free (i.e. adjust tx_free_threshold of i40e driver) to 79 balance consumer and producer. 80 81 * Guest memory should be backended with huge pages to achieve better 82 performance. Using 1G page size is the best. 83 84 When dequeue zero copy is enabled, the guest phys address and host phys 85 address mapping has to be established. Using non-huge pages means far 86 more page segments. To make it simple, DPDK vhost does a linear search 87 of those segments, thus the fewer the segments, the quicker we will get 88 the mapping. NOTE: we may speed it by using tree searching in future. 89 90 * zero copy can not work when using vfio-pci with iommu mode currently, this 91 is because we don't setup iommu dma mapping for guest memory. If you have 92 to use vfio-pci driver, please insert vfio-pci kernel module in noiommu 93 mode. 94 95 * The consumer of zero copy mbufs should consume these mbufs as soon as 96 possible, otherwise it may block the operations in vhost. 97 98 - ``RTE_VHOST_USER_IOMMU_SUPPORT`` 99 100 IOMMU support will be enabled when this flag is set. It is disabled by 101 default. 102 103 Enabling this flag makes possible to use guest vIOMMU to protect vhost 104 from accessing memory the virtio device isn't allowed to, when the feature 105 is negotiated and an IOMMU device is declared. 106 107 However, this feature enables vhost-user's reply-ack protocol feature, 108 which implementation is buggy in Qemu v2.7.0-v2.9.0 when doing multiqueue. 109 Enabling this flag with these Qemu version results in Qemu being blocked 110 when multiple queue pairs are declared. 111 112 - ``RTE_VHOST_USER_POSTCOPY_SUPPORT`` 113 114 Postcopy live-migration support will be enabled when this flag is set. 115 It is disabled by default. 116 117 Enabling this flag should only be done when the calling application does 118 not pre-fault the guest shared memory, otherwise migration would fail. 119 120* ``rte_vhost_driver_set_features(path, features)`` 121 122 This function sets the feature bits the vhost-user driver supports. The 123 vhost-user driver could be vhost-user net, yet it could be something else, 124 say, vhost-user SCSI. 125 126* ``rte_vhost_driver_callback_register(path, vhost_device_ops)`` 127 128 This function registers a set of callbacks, to let DPDK applications take 129 the appropriate action when some events happen. The following events are 130 currently supported: 131 132 * ``new_device(int vid)`` 133 134 This callback is invoked when a virtio device becomes ready. ``vid`` 135 is the vhost device ID. 136 137 * ``destroy_device(int vid)`` 138 139 This callback is invoked when a virtio device is paused or shut down. 140 141 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)`` 142 143 This callback is invoked when a specific queue's state is changed, for 144 example to enabled or disabled. 145 146 * ``features_changed(int vid, uint64_t features)`` 147 148 This callback is invoked when the features is changed. For example, 149 ``VHOST_F_LOG_ALL`` will be set/cleared at the start/end of live 150 migration, respectively. 151 152 * ``new_connection(int vid)`` 153 154 This callback is invoked on new vhost-user socket connection. If DPDK 155 acts as the server the device should not be deleted before 156 ``destroy_connection`` callback is received. 157 158 * ``destroy_connection(int vid)`` 159 160 This callback is invoked when vhost-user socket connection is closed. 161 It indicates that device with id ``vid`` is no longer in use and can be 162 safely deleted. 163 164* ``rte_vhost_driver_disable/enable_features(path, features))`` 165 166 This function disables/enables some features. For example, it can be used to 167 disable mergeable buffers and TSO features, which both are enabled by 168 default. 169 170* ``rte_vhost_driver_start(path)`` 171 172 This function triggers the vhost-user negotiation. It should be invoked at 173 the end of initializing a vhost-user driver. 174 175* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)`` 176 177 Transmits (enqueues) ``count`` packets from host to guest. 178 179* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)`` 180 181 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``. 182 183* ``rte_vhost_crypto_create(vid, cryptodev_id, sess_mempool, socket_id)`` 184 185 As an extension of new_device(), this function adds virtio-crypto workload 186 acceleration capability to the device. All crypto workload is processed by 187 DPDK cryptodev with the device ID of ``cryptodev_id``. 188 189* ``rte_vhost_crypto_free(vid)`` 190 191 Frees the memory and vhost-user message handlers created in 192 rte_vhost_crypto_create(). 193 194* ``rte_vhost_crypto_fetch_requests(vid, queue_id, ops, nb_ops)`` 195 196 Receives (dequeues) ``nb_ops`` virtio-crypto requests from guest, parses 197 them to DPDK Crypto Operations, and fills the ``ops`` with parsing results. 198 199* ``rte_vhost_crypto_finalize_requests(queue_id, ops, nb_ops)`` 200 201 After the ``ops`` are dequeued from Cryptodev, finalizes the jobs and 202 notifies the guest(s). 203 204* ``rte_vhost_crypto_set_zero_copy(vid, option)`` 205 206 Enable or disable zero copy feature of the vhost crypto backend. 207 208Vhost-user Implementations 209-------------------------- 210 211Vhost-user uses Unix domain sockets for passing messages. This means the DPDK 212vhost-user implementation has two options: 213 214* DPDK vhost-user acts as the server. 215 216 DPDK will create a Unix domain socket server file and listen for 217 connections from the frontend. 218 219 Note, this is the default mode, and the only mode before DPDK v16.07. 220 221 222* DPDK vhost-user acts as the client. 223 224 Unlike the server mode, this mode doesn't create the socket file; 225 it just tries to connect to the server (which responses to create the 226 file instead). 227 228 When the DPDK vhost-user application restarts, DPDK vhost-user will try to 229 connect to the server again. This is how the "reconnect" feature works. 230 231 .. Note:: 232 * The "reconnect" feature requires **QEMU v2.7** (or above). 233 234 * The vhost supported features must be exactly the same before and 235 after the restart. For example, if TSO is disabled and then enabled, 236 nothing will work and issues undefined might happen. 237 238No matter which mode is used, once a connection is established, DPDK 239vhost-user will start receiving and processing vhost messages from QEMU. 240 241For messages with a file descriptor, the file descriptor can be used directly 242in the vhost process as it is already installed by the Unix domain socket. 243 244The supported vhost messages are: 245 246* ``VHOST_SET_MEM_TABLE`` 247* ``VHOST_SET_VRING_KICK`` 248* ``VHOST_SET_VRING_CALL`` 249* ``VHOST_SET_LOG_FD`` 250* ``VHOST_SET_VRING_ERR`` 251 252For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each 253memory region and its file descriptor in the ancillary data of the message. 254The file descriptor is used to map that region. 255 256``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into 257the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove 258the vhost device from the data plane. 259 260When the socket connection is closed, vhost will destroy the device. 261 262Guest memory requirement 263------------------------ 264 265* Memory pre-allocation 266 267 For non-zerocopy, guest memory pre-allocation is not a must. This can help 268 save of memory. If users really want the guest memory to be pre-allocated 269 (e.g., for performance reason), we can add option ``-mem-prealloc`` when 270 starting QEMU. Or, we can lock all memory at vhost side which will force 271 memory to be allocated when mmap at vhost side; option --mlockall in 272 ovs-dpdk is an example in hand. 273 274 For zerocopy, we force the VM memory to be pre-allocated at vhost lib when 275 mapping the guest memory; and also we need to lock the memory to prevent 276 pages being swapped out to disk. 277 278* Memory sharing 279 280 Make sure ``share=on`` QEMU option is given. vhost-user will not work with 281 a QEMU version without shared memory mapping. 282 283Vhost supported vSwitch reference 284--------------------------------- 285 286For more vhost details and how to support vhost in vSwitch, please refer to 287the vhost example in the DPDK Sample Applications Guide. 288 289Vhost data path acceleration (vDPA) 290----------------------------------- 291 292vDPA supports selective datapath in vhost-user lib by enabling virtio ring 293compatible devices to serve virtio driver directly for datapath acceleration. 294 295``rte_vhost_driver_attach_vdpa_device`` is used to configure the vhost device 296with accelerated backend. 297 298Also vhost device capabilities are made configurable to adopt various devices. 299Such capabilities include supported features, protocol features, queue number. 300 301Finally, a set of device ops is defined for device specific operations: 302 303* ``get_queue_num`` 304 305 Called to get supported queue number of the device. 306 307* ``get_features`` 308 309 Called to get supported features of the device. 310 311* ``get_protocol_features`` 312 313 Called to get supported protocol features of the device. 314 315* ``dev_conf`` 316 317 Called to configure the actual device when the virtio device becomes ready. 318 319* ``dev_close`` 320 321 Called to close the actual device when the virtio device is stopped. 322 323* ``set_vring_state`` 324 325 Called to change the state of the vring in the actual device when vring state 326 changes. 327 328* ``set_features`` 329 330 Called to set the negotiated features to device. 331 332* ``migration_done`` 333 334 Called to allow the device to response to RARP sending. 335 336* ``get_vfio_group_fd`` 337 338 Called to get the VFIO group fd of the device. 339 340* ``get_vfio_device_fd`` 341 342 Called to get the VFIO device fd of the device. 343 344* ``get_notify_area`` 345 346 Called to get the notify area info of the queue. 347