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 - ``RTE_VHOST_USER_IOMMU_SUPPORT`` 87 88 IOMMU support will be enabled when this flag is set. It is disabled by 89 default. 90 91 Enabling this flag makes possible to use guest vIOMMU to protect vhost 92 from accessing memory the virtio device isn't allowed to, when the feature 93 is negotiated and an IOMMU device is declared. 94 95 However, this feature enables vhost-user's reply-ack protocol feature, 96 which implementation is buggy in Qemu v2.7.0-v2.9.0 when doing multiqueue. 97 Enabling this flag with these Qemu version results in Qemu being blocked 98 when multiple queue pairs are declared. 99 100* ``rte_vhost_driver_set_features(path, features)`` 101 102 This function sets the feature bits the vhost-user driver supports. The 103 vhost-user driver could be vhost-user net, yet it could be something else, 104 say, vhost-user SCSI. 105 106* ``rte_vhost_driver_callback_register(path, vhost_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 device becomes ready. ``vid`` 115 is the vhost device ID. 116 117 * ``destroy_device(int vid)`` 118 119 This callback is invoked when a virtio device is paused or shut down. 120 121 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)`` 122 123 This callback is invoked when a specific queue's state is changed, for 124 example to enabled or disabled. 125 126 * ``features_changed(int vid, uint64_t features)`` 127 128 This callback is invoked when the features is changed. For example, 129 ``VHOST_F_LOG_ALL`` will be set/cleared at the start/end of live 130 migration, respectively. 131 132 * ``new_connection(int vid)`` 133 134 This callback is invoked on new vhost-user socket connection. If DPDK 135 acts as the server the device should not be deleted before 136 ``destroy_connection`` callback is received. 137 138 * ``destroy_connection(int vid)`` 139 140 This callback is invoked when vhost-user socket connection is closed. 141 It indicates that device with id ``vid`` is no longer in use and can be 142 safely deleted. 143 144* ``rte_vhost_driver_disable/enable_features(path, features))`` 145 146 This function disables/enables some features. For example, it can be used to 147 disable mergeable buffers and TSO features, which both are enabled by 148 default. 149 150* ``rte_vhost_driver_start(path)`` 151 152 This function triggers the vhost-user negotiation. It should be invoked at 153 the end of initializing a vhost-user driver. 154 155* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)`` 156 157 Transmits (enqueues) ``count`` packets from host to guest. 158 159* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)`` 160 161 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``. 162 163Vhost-user Implementations 164-------------------------- 165 166Vhost-user uses Unix domain sockets for passing messages. This means the DPDK 167vhost-user implementation has two options: 168 169* DPDK vhost-user acts as the server. 170 171 DPDK will create a Unix domain socket server file and listen for 172 connections from the frontend. 173 174 Note, this is the default mode, and the only mode before DPDK v16.07. 175 176 177* DPDK vhost-user acts as the client. 178 179 Unlike the server mode, this mode doesn't create the socket file; 180 it just tries to connect to the server (which responses to create the 181 file instead). 182 183 When the DPDK vhost-user application restarts, DPDK vhost-user will try to 184 connect to the server again. This is how the "reconnect" feature works. 185 186 .. Note:: 187 * The "reconnect" feature requires **QEMU v2.7** (or above). 188 189 * The vhost supported features must be exactly the same before and 190 after the restart. For example, if TSO is disabled and then enabled, 191 nothing will work and issues undefined might happen. 192 193No matter which mode is used, once a connection is established, DPDK 194vhost-user will start receiving and processing vhost messages from QEMU. 195 196For messages with a file descriptor, the file descriptor can be used directly 197in the vhost process as it is already installed by the Unix domain socket. 198 199The supported vhost messages are: 200 201* ``VHOST_SET_MEM_TABLE`` 202* ``VHOST_SET_VRING_KICK`` 203* ``VHOST_SET_VRING_CALL`` 204* ``VHOST_SET_LOG_FD`` 205* ``VHOST_SET_VRING_ERR`` 206 207For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each 208memory region and its file descriptor in the ancillary data of the message. 209The file descriptor is used to map that region. 210 211``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into 212the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove 213the vhost device from the data plane. 214 215When the socket connection is closed, vhost will destroy the device. 216 217Guest memory requirement 218------------------------ 219 220* Memory pre-allocation 221 222 For non-zerocopy, guest memory pre-allocation is not a must. This can help 223 save of memory. If users really want the guest memory to be pre-allocated 224 (e.g., for performance reason), we can add option ``-mem-prealloc`` when 225 starting QEMU. Or, we can lock all memory at vhost side which will force 226 memory to be allocated when mmap at vhost side; option --mlockall in 227 ovs-dpdk is an example in hand. 228 229 For zerocopy, we force the VM memory to be pre-allocated at vhost lib when 230 mapping the guest memory; and also we need to lock the memory to prevent 231 pages being swapped out to disk. 232 233* Memory sharing 234 235 Make sure ``share=on`` QEMU option is given. vhost-user will not work with 236 a QEMU version without shared memory mapping. 237 238Vhost supported vSwitch reference 239--------------------------------- 240 241For more vhost details and how to support vhost in vSwitch, please refer to 242the vhost example in the DPDK Sample Applications Guide. 243