[dpdk-dev] [PATCH v2] doc: update vhost guide
yuanhan.liu at linux.intel.com
Mon Jun 27 07:20:00 CEST 2016
Mainly on updating vhost-user part: we now support client mode.
Also refine some words, and add a bit more explanation.
And made an emphatic statement that you are suggested to use vhost-user
instead of vhost-cuse, because we have enhanced vhost-user a lot since
v2.2 (Actually, I doubt there are any people still using vhost-cuse)
[John McNamara: rewords, better formats]
Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
doc/guides/prog_guide/vhost_lib.rst | 223 +++++++++++++++++++++++++-----------
1 file changed, 156 insertions(+), 67 deletions(-)
diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index 48e1fff..14d5e67 100644
@@ -1,5 +1,5 @@
.. BSD LICENSE
- Copyright(c) 2010-2014 Intel Corporation. All rights reserved.
+ Copyright(c) 2010-2016 Intel Corporation. All rights reserved.
All rights reserved.
Redistribution and use in source and binary forms, with or without
@@ -31,105 +31,194 @@
-The vhost library implements a user space vhost driver. It supports both vhost-cuse
-(cuse: user space character device) and vhost-user(user space socket server).
-It also creates, manages and destroys vhost devices for corresponding virtio
-devices in the guest. Vhost supported vSwitch could register callbacks to this
-library, which will be called when a vhost device is activated or deactivated
-by guest virtual machine.
+The vhost library implements a user space virtio net server allowing the user
+to manipulate the virtio ring directly. In another words, it allows the user
+to fetch/put packets from/to the VM virtio net device. To achieve this, a
+vhost library should be able to:
+* Access the guest memory:
+ For QEMU, this is done by using the ``-object memory-backend-file,share=on,...``
+ option. Which means QEMU will create a file to serve as the guest RAM.
+ The ``share=on`` option allows another process to map that file, which
+ means it can access the guest RAM.
+* Know all the necessary information about the vring:
+ Information such as where the available ring is stored. Vhost defines some
+ messages to tell the backend all the information it needs to know how to
+ manipulate the vring.
+Currently, there are two ways to pass these messages and as a result there are
+two Vhost implementations in DPDK: *vhost-cuse* (where the character devices
+are in user space) and *vhost-user*.
+Vhost-cuse creates a user space character device and hook to a function ioctl,
+so that all ioctl commands that are sent from the frontend (QEMU) will be
+captured and handled.
+Vhost-user creates a Unix domain socket file through which messages are
+ Since DPDK v2.2, the majority of the development effort has gone into
+ enhancing vhost-user, such as multiple queue, live migration, and
+ reconnect. Thus, it is strongly advised to use vhost-user instead of
Vhost API Overview
-* Vhost driver registration
+The following is an overview of the Vhost API functions:
+* ``rte_vhost_driver_register(path, flags)``
+ This function registers a vhost driver into the system. For vhost-cuse, a
+ ``/dev/path`` character device file will be created. For vhost-user server
+ mode, a Unix domain socket file ``path`` will be created.
+ Currently two flags are supported (these are valid for vhost-user only):
+ - ``RTE_VHOST_USER_CLIENT``
+ DPDK vhost-user will act as the client when this flag is given. See below
+ for an explanation.
+ - ``RTE_VHOST_USER_NO_RECONNECT``
+ When DPDK vhost-user acts as the client it will keep trying to reconnect
+ to the server (QEMU) until it succeeds. This is useful in two cases:
+ * When QEMU is not started yet.
+ * When QEMU restarts (for example due to a guest OS reboot).
+ This reconnect option is enabled by default. However, it can be turned off
+ by setting this flag.
- rte_vhost_driver_register registers the vhost driver into the system.
- For vhost-cuse, character device file will be created under the /dev directory.
- Character device name is specified as the parameter.
- For vhost-user, a Unix domain socket server will be created with the parameter as
- the local socket path.
-* Vhost session start
+ This function starts the vhost session loop to handle vhost messages. It
+ starts an infinite loop, therefore it should be called in a dedicated
- rte_vhost_driver_session_start starts the vhost session loop.
- Vhost session is an infinite blocking loop.
- Put the session in a dedicate DPDK thread.
-* Callback register
+ This function registers a set of callbacks, to let DPDK applications take
+ the appropriate action when some events happen. The following events are
+ currently supported:
- Vhost supported vSwitch could call rte_vhost_driver_callback_register to
- register two callbacks, new_destory and destroy_device.
- When virtio device is activated or deactivated by guest virtual machine,
- the callback will be called, then vSwitch could put the device onto data
- core or remove the device from data core by setting or unsetting
- VIRTIO_DEV_RUNNING on the device flags.
+ * ``new_device(int vid)``
-* Read/write packets from/to guest virtual machine
+ This callback is invoked when a virtio net device becomes ready. ``vid``
+ is the virtio net device ID.
- rte_vhost_enqueue_burst transmit host packets to guest.
- rte_vhost_dequeue_burst receives packets from guest.
+ * ``destroy_device(int vid)``
-* Feature enable/disable
+ This callback is invoked when a virtio net device shuts down (or when the
+ vhost connection is broken).
- Now one negotiate-able feature in vhost is merge-able.
- vSwitch could enable/disable this feature for performance consideration.
+ * ``vring_state_changed(int vid, uint16_t queue_id, int enable)``
+ This callback is invoked when a specific queue's state is changed, for
+ example to enabled or disabled.
-Vhost cuse implementation
+* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)``
+ Transmits (enqueues) ``count`` packets from host to guest.
+* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)``
+ Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``.
+ This function disables/enables some features. For example, it can be used to
+ disable mergeable buffers and TSO features, which both are enabled by
When vSwitch registers the vhost driver, it will register a cuse device driver
into the system and creates a character device file. This cuse driver will
-receive vhost open/release/IOCTL message from QEMU simulator.
+receive vhost open/release/IOCTL messages from the QEMU simulator.
-When the open call is received, vhost driver will create a vhost device for the
-virtio device in the guest.
+When the open call is received, the vhost driver will create a vhost device
+for the virtio device in the guest.
-When VHOST_SET_MEM_TABLE IOCTL is received, vhost searches the memory region
-to find the starting user space virtual address that maps the memory of guest
-virtual machine. Through this virtual address and the QEMU pid, vhost could
-find the file QEMU uses to map the guest memory. Vhost maps this file into its
-address space, in this way vhost could fully access the guest physical memory,
-which means vhost could access the shared virtio ring and the guest physical
-address specified in the entry of the ring.
+When the ``VHOST_SET_MEM_TABLE`` ioctl is received, vhost searches the memory
+region to find the starting user space virtual address that maps the memory of
+the guest virtual machine. Through this virtual address and the QEMU pid,
+vhost can find the file QEMU uses to map the guest memory. Vhost maps this
+file into its address space, in this way vhost can fully access the guest
+physical memory, which means vhost could access the shared virtio ring and the
+guest physical address specified in the entry of the ring.
The guest virtual machine tells the vhost whether the virtio device is ready
-for processing or is de-activated through VHOST_NET_SET_BACKEND message.
-The registered callback from vSwitch will be called.
+for processing or is de-activated through the ``VHOST_NET_SET_BACKEND``
+message. The registered callback from vSwitch will be called.
-When the release call is released, vhost will destroy the device.
+When the release call is made, vhost will destroy the device.
-Vhost user implementation
-When vSwitch registers a vhost driver, it will create a Unix domain socket server
-into the system. This server will listen for a connection and process the vhost message from
-When there is a new socket connection, it means a new virtio device has been created in
-the guest virtual machine, and the vhost driver will create a vhost device for this virtio device.
+Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
+vhost-user implementation has two options:
+* DPDK vhost-user acts as the server.
+ DPDK will create a Unix domain socket server file and listen for
+ connections from the frontend.
+ Note, this is the default mode, and the only mode before DPDK v16.07.
+* DPDK vhost-user acts as the client.
+ Unlike the server mode, this mode doesn't create the socket file;
+ it just tries to connect to the server (which responses to create the
+ file instead).
+ When the DPDK vhost-user application restarts, DPDK vhost-user will try to
+ connect to the server again. This is how the "reconnect" feature works.
+ Note: the "reconnect" feature requires **QEMU v2.7** (or above).
+No matter which mode is used, once a connection is established, DPDK
+vhost-user will start receiving and processing vhost messages from QEMU.
+For messages with a file descriptor, the file descriptor can be used directly
+in the vhost process as it is already installed by the Unix domain socket.
-For messages with a file descriptor, the file descriptor could be directly used in the vhost
-process as it is already installed by Unix domain socket.
+The supported vhost messages are:
- * VHOST_SET_MEM_TABLE
- * VHOST_SET_VRING_KICK
- * VHOST_SET_VRING_CALL
- * VHOST_SET_LOG_FD
- * VHOST_SET_VRING_ERR
-For VHOST_SET_MEM_TABLE message, QEMU will send us information for each memory region and its
-file descriptor in the ancillary data of the message. The fd is used to map that region.
+For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each
+memory region and its file descriptor in the ancillary data of the message.
+The file descriptor is used to map that region.
-There is no VHOST_NET_SET_BACKEND message as in vhost cuse to signal us whether virtio device
-is ready or should be stopped.
-VHOST_SET_VRING_KICK is used as the signal to put the vhost device onto data plane.
-VHOST_GET_VRING_BASE is used as the signal to remove vhost device from data plane.
+There is no ``VHOST_NET_SET_BACKEND`` message as in vhost-cuse to signal
+whether the virtio device is ready or stopped. Instead,
+``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into
+the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove
+the vhost device from the data plane.
When the socket connection is closed, vhost will destroy the device.
Vhost supported vSwitch reference
-For more vhost details and how to support vhost in vSwitch, please refer to vhost example in the
-DPDK Sample Applications Guide.
+For more vhost details and how to support vhost in vSwitch, please refer to
+the vhost example in the DPDK Sample Applications Guide.
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