1 .. SPDX-License-Identifier: BSD-3-Clause
2 Copyright(c) 2010-2016 Intel Corporation.
7 The vhost library implements a user space virtio net server allowing the user
8 to manipulate the virtio ring directly. In another words, it allows the user
9 to fetch/put packets from/to the VM virtio net device. To achieve this, a
10 vhost library should be able to:
12 * Access the guest memory:
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.
19 * Know all the necessary information about the vring:
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.
29 The following is an overview of some key Vhost API functions:
31 * ``rte_vhost_driver_register(path, flags)``
33 This function registers a vhost driver into the system. ``path`` specifies
34 the Unix domain socket file path.
36 Currently supported flags are:
38 - ``RTE_VHOST_USER_CLIENT``
40 DPDK vhost-user will act as the client when this flag is given. See below
43 - ``RTE_VHOST_USER_NO_RECONNECT``
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:
48 * When QEMU is not started yet.
49 * When QEMU restarts (for example due to a guest OS reboot).
51 This reconnect option is enabled by default. However, it can be turned off
54 - ``RTE_VHOST_USER_DEQUEUE_ZERO_COPY``
56 Dequeue zero copy will be enabled when this flag is set. It is disabled by
59 There are some truths (including limitations) you might want to know while
62 * zero copy is not good for small packets (typically for packet size below
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).
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.
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
77 * Guest memory should be backended with huge pages to achieve better
78 performance. Using 1G page size is the best.
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.
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
91 - ``RTE_VHOST_USER_IOMMU_SUPPORT``
93 IOMMU support will be enabled when this flag is set. It is disabled by
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.
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.
105 * ``rte_vhost_driver_set_features(path, features)``
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.
111 * ``rte_vhost_driver_callback_register(path, vhost_device_ops)``
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
117 * ``new_device(int vid)``
119 This callback is invoked when a virtio device becomes ready. ``vid``
120 is the vhost device ID.
122 * ``destroy_device(int vid)``
124 This callback is invoked when a virtio device is paused or shut down.
126 * ``vring_state_changed(int vid, uint16_t queue_id, int enable)``
128 This callback is invoked when a specific queue's state is changed, for
129 example to enabled or disabled.
131 * ``features_changed(int vid, uint64_t features)``
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.
137 * ``new_connection(int vid)``
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.
143 * ``destroy_connection(int vid)``
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
149 * ``rte_vhost_driver_disable/enable_features(path, features))``
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
155 * ``rte_vhost_driver_start(path)``
157 This function triggers the vhost-user negotiation. It should be invoked at
158 the end of initializing a vhost-user driver.
160 * ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)``
162 Transmits (enqueues) ``count`` packets from host to guest.
164 * ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)``
166 Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``.
168 * ``rte_vhost_crypto_create(vid, cryptodev_id, sess_mempool, socket_id)``
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``.
174 * ``rte_vhost_crypto_free(vid)``
176 Frees the memory and vhost-user message handlers created in
177 rte_vhost_crypto_create().
179 * ``rte_vhost_crypto_fetch_requests(vid, queue_id, ops, nb_ops)``
181 Receives (dequeues) ``nb_ops`` virtio-crypto requests from guest, parses
182 them to DPDK Crypto Operations, and fills the ``ops`` with parsing results.
184 * ``rte_vhost_crypto_finalize_requests(queue_id, ops, nb_ops)``
186 After the ``ops`` are dequeued from Cryptodev, finalizes the jobs and
187 notifies the guest(s).
189 * ``rte_vhost_crypto_set_zero_copy(vid, option)``
191 Enable or disable zero copy feature of the vhost crypto backend.
193 Vhost-user Implementations
194 --------------------------
196 Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
197 vhost-user implementation has two options:
199 * DPDK vhost-user acts as the server.
201 DPDK will create a Unix domain socket server file and listen for
202 connections from the frontend.
204 Note, this is the default mode, and the only mode before DPDK v16.07.
207 * DPDK vhost-user acts as the client.
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
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.
217 * The "reconnect" feature requires **QEMU v2.7** (or above).
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.
223 No matter which mode is used, once a connection is established, DPDK
224 vhost-user will start receiving and processing vhost messages from QEMU.
226 For messages with a file descriptor, the file descriptor can be used directly
227 in the vhost process as it is already installed by the Unix domain socket.
229 The supported vhost messages are:
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``
237 For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each
238 memory region and its file descriptor in the ancillary data of the message.
239 The file descriptor is used to map that region.
241 ``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into
242 the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove
243 the vhost device from the data plane.
245 When the socket connection is closed, vhost will destroy the device.
247 Guest memory requirement
248 ------------------------
250 * Memory pre-allocation
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.
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.
265 Make sure ``share=on`` QEMU option is given. vhost-user will not work with
266 a QEMU version without shared memory mapping.
268 Vhost supported vSwitch reference
269 ---------------------------------
271 For more vhost details and how to support vhost in vSwitch, please refer to
272 the vhost example in the DPDK Sample Applications Guide.