doc: update vhost guide

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@linux.intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>

diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index 48e1fff..14d5e67 100644 (file)
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -1,5 +1,5 @@
 ..  BSD LICENSE
 ..  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
     All rights reserved.
 
     Redistribution and use in source and binary forms, with or without


@@ -31,105 +31,194 @@
 Vhost Library
 =============
 
 Vhost Library
 =============
 

-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
+passed.
+
+.. Note::
+
+   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-cuse.
+

 
 Vhost API Overview
 ------------------
 
 
 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.
+* ``rte_vhost_driver_session_start()``

 
 

-*   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
+  thread.

 
 

-      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.
+* ``rte_vhost_driver_callback_register(virtio_net_device_ops)``

 
 

-*   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)``

 
 

-Vhost Implementation
---------------------
+    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``.
+
+* ``rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)``
+
+  This function disables/enables some features. For example, it can be used to
+  disable mergeable buffers and TSO features, which both are enabled by
+  default.
+
+
+Vhost Implementations
+---------------------
+
+Vhost-cuse implementation

 ~~~~~~~~~~~~~~~~~~~~~~~~~
 ~~~~~~~~~~~~~~~~~~~~~~~~~

+

 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
 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
 
 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
+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
-QEMU simulator.

 
 

-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
+* ``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
 ---------------------------------
 
 
 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.