vhost_lib.rst revision 7b53c036
1..  BSD LICENSE
2    Copyright(c) 2010-2016 Intel Corporation. All rights reserved.
3    All rights reserved.
4
5    Redistribution and use in source and binary forms, with or without
6    modification, are permitted provided that the following conditions
7    are met:
8
9    * Redistributions of source code must retain the above copyright
10    notice, this list of conditions and the following disclaimer.
11    * Redistributions in binary form must reproduce the above copyright
12    notice, this list of conditions and the following disclaimer in
13    the documentation and/or other materials provided with the
14    distribution.
15    * Neither the name of Intel Corporation nor the names of its
16    contributors may be used to endorse or promote products derived
17    from this software without specific prior written permission.
18
19    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
22    A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
23    OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
24    SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
25    LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
26    DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
27    THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
28    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
29    OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
30
31Vhost Library
32=============
33
34The vhost library implements a user space virtio net server allowing the user
35to manipulate the virtio ring directly. In another words, it allows the user
36to fetch/put packets from/to the VM virtio net device. To achieve this, a
37vhost 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 to tell the backend all the information it needs to know how to
50  manipulate the vring.
51
52Currently, there are two ways to pass these messages and as a result there are
53two Vhost implementations in DPDK: *vhost-cuse* (where the character devices
54are in user space) and *vhost-user*.
55
56Vhost-cuse creates a user space character device and hook to a function ioctl,
57so that all ioctl commands that are sent from the frontend (QEMU) will be
58captured and handled.
59
60Vhost-user creates a Unix domain socket file through which messages are
61passed.
62
63.. Note::
64
65   Since DPDK v2.2, the majority of the development effort has gone into
66   enhancing vhost-user, such as multiple queue, live migration, and
67   reconnect. Thus, it is strongly advised to use vhost-user instead of
68   vhost-cuse.
69
70
71Vhost API Overview
72------------------
73
74The following is an overview of the Vhost API functions:
75
76* ``rte_vhost_driver_register(path, flags)``
77
78  This function registers a vhost driver into the system. For vhost-cuse, a
79  ``/dev/path`` character device file will be created. For vhost-user server
80  mode, a Unix domain socket file ``path`` will be created.
81
82  Currently two flags are supported (these are valid for vhost-user only):
83
84  - ``RTE_VHOST_USER_CLIENT``
85
86    DPDK vhost-user will act as the client when this flag is given. See below
87    for an explanation.
88
89  - ``RTE_VHOST_USER_NO_RECONNECT``
90
91    When DPDK vhost-user acts as the client it will keep trying to reconnect
92    to the server (QEMU) until it succeeds. This is useful in two cases:
93
94    * When QEMU is not started yet.
95    * When QEMU restarts (for example due to a guest OS reboot).
96
97    This reconnect option is enabled by default. However, it can be turned off
98    by setting this flag.
99
100* ``rte_vhost_driver_session_start()``
101
102  This function starts the vhost session loop to handle vhost messages. It
103  starts an infinite loop, therefore it should be called in a dedicated
104  thread.
105
106* ``rte_vhost_driver_callback_register(virtio_net_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 net device becomes ready. ``vid``
115    is the virtio net device ID.
116
117  * ``destroy_device(int vid)``
118
119    This callback is invoked when a virtio net device shuts down (or when the
120    vhost connection is broken).
121
122  * ``vring_state_changed(int vid, uint16_t queue_id, int enable)``
123
124    This callback is invoked when a specific queue's state is changed, for
125    example to enabled or disabled.
126
127* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)``
128
129  Transmits (enqueues) ``count`` packets from host to guest.
130
131* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)``
132
133  Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``.
134
135* ``rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)``
136
137  This function disables/enables some features. For example, it can be used to
138  disable mergeable buffers and TSO features, which both are enabled by
139  default.
140
141
142Vhost Implementations
143---------------------
144
145Vhost-cuse implementation
146~~~~~~~~~~~~~~~~~~~~~~~~~
147
148When vSwitch registers the vhost driver, it will register a cuse device driver
149into the system and creates a character device file. This cuse driver will
150receive vhost open/release/IOCTL messages from the QEMU simulator.
151
152When the open call is received, the vhost driver will create a vhost device
153for the virtio device in the guest.
154
155When the ``VHOST_SET_MEM_TABLE`` ioctl is received, vhost searches the memory
156region to find the starting user space virtual address that maps the memory of
157the guest virtual machine. Through this virtual address and the QEMU pid,
158vhost can find the file QEMU uses to map the guest memory. Vhost maps this
159file into its address space, in this way vhost can fully access the guest
160physical memory, which means vhost could access the shared virtio ring and the
161guest physical address specified in the entry of the ring.
162
163The guest virtual machine tells the vhost whether the virtio device is ready
164for processing or is de-activated through the ``VHOST_NET_SET_BACKEND``
165message. The registered callback from vSwitch will be called.
166
167When the release call is made, vhost will destroy the device.
168
169Vhost-user implementation
170~~~~~~~~~~~~~~~~~~~~~~~~~
171
172Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
173vhost-user implementation has two options:
174
175* DPDK vhost-user acts as the server.
176
177  DPDK will create a Unix domain socket server file and listen for
178  connections from the frontend.
179
180  Note, this is the default mode, and the only mode before DPDK v16.07.
181
182
183* DPDK vhost-user acts as the client.
184
185  Unlike the server mode, this mode doesn't create the socket file;
186  it just tries to connect to the server (which responses to create the
187  file instead).
188
189  When the DPDK vhost-user application restarts, DPDK vhost-user will try to
190  connect to the server again. This is how the "reconnect" feature works.
191
192  .. Note::
193     * The "reconnect" feature requires **QEMU v2.7** (or above).
194
195     * The vhost supported features must be exactly the same before and
196       after the restart. For example, if TSO is disabled and then enabled,
197       nothing will work and issues undefined might happen.
198
199No matter which mode is used, once a connection is established, DPDK
200vhost-user will start receiving and processing vhost messages from QEMU.
201
202For messages with a file descriptor, the file descriptor can be used directly
203in the vhost process as it is already installed by the Unix domain socket.
204
205The supported vhost messages are:
206
207* ``VHOST_SET_MEM_TABLE``
208* ``VHOST_SET_VRING_KICK``
209* ``VHOST_SET_VRING_CALL``
210* ``VHOST_SET_LOG_FD``
211* ``VHOST_SET_VRING_ERR``
212
213For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each
214memory region and its file descriptor in the ancillary data of the message.
215The file descriptor is used to map that region.
216
217There is no ``VHOST_NET_SET_BACKEND`` message as in vhost-cuse to signal
218whether the virtio device is ready or stopped. Instead,
219``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into
220the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove
221the vhost device from the data plane.
222
223When the socket connection is closed, vhost will destroy the device.
224
225Vhost supported vSwitch reference
226---------------------------------
227
228For more vhost details and how to support vhost in vSwitch, please refer to
229the vhost example in the DPDK Sample Applications Guide.
230