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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:
39 * Access the guest memory:
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.
46 * Know all the necessary information about the vring:
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.
56 The following is an overview of the Vhost API functions:
58 * ``rte_vhost_driver_register(path, flags)``
60 This function registers a vhost driver into the system. ``path`` specifies
61 the Unix domain socket file path.
63 Currently supported flags are:
65 - ``RTE_VHOST_USER_CLIENT``
67 DPDK vhost-user will act as the client when this flag is given. See below
70 - ``RTE_VHOST_USER_NO_RECONNECT``
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:
75 * When QEMU is not started yet.
76 * When QEMU restarts (for example due to a guest OS reboot).
78 This reconnect option is enabled by default. However, it can be turned off
81 - ``RTE_VHOST_USER_DEQUEUE_ZERO_COPY``
83 Dequeue zero copy will be enabled when this flag is set. It is disabled by
86 There are some truths (including limitations) you might want to know while
89 * zero copy is not good for small packets (typically for packet size below
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).
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.
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
104 * Guest memory should be backended with huge pages to achieve better
105 performance. Using 1G page size is the best.
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.
113 * ``rte_vhost_driver_session_start()``
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
119 * ``rte_vhost_driver_callback_register(virtio_net_device_ops)``
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
125 * ``new_device(int vid)``
127 This callback is invoked when a virtio net device becomes ready. ``vid``
128 is the virtio net device ID.
130 * ``destroy_device(int vid)``
132 This callback is invoked when a virtio net device shuts down (or when the
133 vhost connection is broken).
135 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)``
137 This callback is invoked when a specific queue's state is changed, for
138 example to enabled or disabled.
140 * ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)``
142 Transmits (enqueues) ``count`` packets from host to guest.
144 * ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)``
146 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``.
148 * ``rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)``
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
155 Vhost-user Implementations
156 --------------------------
158 Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
159 vhost-user implementation has two options:
161 * DPDK vhost-user acts as the server.
163 DPDK will create a Unix domain socket server file and listen for
164 connections from the frontend.
166 Note, this is the default mode, and the only mode before DPDK v16.07.
169 * DPDK vhost-user acts as the client.
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
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.
179 * The "reconnect" feature requires **QEMU v2.7** (or above).
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.
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.
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.
191 The supported vhost messages are:
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``
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.
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.
207 When the socket connection is closed, vhost will destroy the device.
209 Vhost supported vSwitch reference
210 ---------------------------------
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.