ceph/src/dpdk/doc/guides/prog_guide/vhost_lib.rst

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  30
  31 Vhost Library
  32 =============
  33
  34 The vhost library implements a user space virtio net server allowing the user
  35 to manipulate the virtio ring directly. In another words, it allows the user
  36 to fetch/put packets from/to the VM virtio net device. To achieve this, a
  37 vhost 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
  53 Vhost API Overview
  54 ------------------
  55
  56 The following is an overview of the 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       The 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_driver_session_start()``
 114
 115   This function starts the vhost session loop to handle vhost messages. It
 116   starts an infinite loop, therefore it should be called in a dedicated
 117   thread.
 118
 119 * ``rte_vhost_driver_callback_register(virtio_net_device_ops)``
 120
 121   This function registers a set of callbacks, to let DPDK applications take
 122   the appropriate action when some events happen. The following events are
 123   currently supported:
 124
 125   * ``new_device(int vid)``
 126
 127     This callback is invoked when a virtio net device becomes ready. ``vid``
 128     is the virtio net device ID.
 129
 130   * ``destroy_device(int vid)``
 131
 132     This callback is invoked when a virtio net device shuts down (or when the
 133     vhost connection is broken).
 134
 135   * ``vring_state_changed(int vid, uint16_t queue_id, int enable)``
 136
 137     This callback is invoked when a specific queue's state is changed, for
 138     example to enabled or disabled.
 139
 140 * ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)``
 141
 142   Transmits (enqueues) ``count`` packets from host to guest.
 143
 144 * ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)``
 145
 146   Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``.
 147
 148 * ``rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)``
 149
 150   This function disables/enables some features. For example, it can be used to
 151   disable mergeable buffers and TSO features, which both are enabled by
 152   default.
 153
 154
 155 Vhost-user Implementations
 156 --------------------------
 157
 158 Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
 159 vhost-user implementation has two options:
 160
 161 * DPDK vhost-user acts as the server.
 162
 163   DPDK will create a Unix domain socket server file and listen for
 164   connections from the frontend.
 165
 166   Note, this is the default mode, and the only mode before DPDK v16.07.
 167
 168
 169 * DPDK vhost-user acts as the client.
 170
 171   Unlike the server mode, this mode doesn't create the socket file;
 172   it just tries to connect to the server (which responses to create the
 173   file instead).
 174
 175   When the DPDK vhost-user application restarts, DPDK vhost-user will try to
 176   connect to the server again. This is how the "reconnect" feature works.
 177
 178   .. Note::
 179      * The "reconnect" feature requires **QEMU v2.7** (or above).
 180
 181      * The vhost supported features must be exactly the same before and
 182        after the restart. For example, if TSO is disabled and then enabled,
 183        nothing will work and issues undefined might happen.
 184
 185 No matter which mode is used, once a connection is established, DPDK
 186 vhost-user will start receiving and processing vhost messages from QEMU.
 187
 188 For messages with a file descriptor, the file descriptor can be used directly
 189 in the vhost process as it is already installed by the Unix domain socket.
 190
 191 The supported vhost messages are:
 192
 193 * ``VHOST_SET_MEM_TABLE``
 194 * ``VHOST_SET_VRING_KICK``
 195 * ``VHOST_SET_VRING_CALL``
 196 * ``VHOST_SET_LOG_FD``
 197 * ``VHOST_SET_VRING_ERR``
 198
 199 For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each
 200 memory region and its file descriptor in the ancillary data of the message.
 201 The file descriptor is used to map that region.
 202
 203 ``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into
 204 the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove
 205 the vhost device from the data plane.
 206
 207 When the socket connection is closed, vhost will destroy the device.
 208
 209 Vhost supported vSwitch reference
 210 ---------------------------------
 211
 212 For more vhost details and how to support vhost in vSwitch, please refer to
 213 the vhost example in the DPDK Sample Applications Guide.