Note Sphinx dependencies in changelog

[deb_dpdk.git] / doc / guides / prog_guide / vhost_lib.rst

diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst
index 6b0c6b2..4f997d4 100644 (file)
--- a/doc/guides/prog_guide/vhost_lib.rst
+++ b/doc/guides/prog_guide/vhost_lib.rst
@@ -46,26 +46,8 @@ vhost library should be able to:
 * Know all the necessary information about the vring:
 
   Information such as where the available ring is stored. Vhost defines some
 * 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.
+  messages (passed through a Unix domain socket file) to tell the backend all
+  the information it needs to know how to manipulate the vring.

 
 
 Vhost API Overview
 
 
 Vhost API Overview


@@ -75,11 +57,10 @@ The following is an overview of the Vhost API functions:
 
 * ``rte_vhost_driver_register(path, flags)``
 
 
 * ``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.
+  This function registers a vhost driver into the system. ``path`` specifies
+  the Unix domain socket file path.

 
 

-  Currently two flags are supported (these are valid for vhost-user only):
+  Currently supported flags are:

 
   - ``RTE_VHOST_USER_CLIENT``
 
 
   - ``RTE_VHOST_USER_CLIENT``
 


@@ -97,6 +78,38 @@ The following is an overview of the Vhost API functions:
     This reconnect option is enabled by default. However, it can be turned off
     by setting this flag.
 
     This reconnect option is enabled by default. However, it can be turned off
     by setting this flag.
 

+  - ``RTE_VHOST_USER_DEQUEUE_ZERO_COPY``
+
+    Dequeue zero copy will be enabled when this flag is set. It is disabled by
+    default.
+
+    There are some truths (including limitations) you might want to know while
+    setting this flag:
+
+    * zero copy is not good for small packets (typically for packet size below
+      512).
+
+    * zero copy is really good for VM2VM case. For iperf between two VMs, the
+      boost could be above 70% (when TSO is enableld).
+
+    * for VM2NIC case, the ``nb_tx_desc`` has to be small enough: <= 64 if virtio
+      indirect feature is not enabled and <= 128 if it is enabled.
+
+      The is because when dequeue zero copy is enabled, guest Tx used vring will
+      be updated only when corresponding mbuf is freed. Thus, the nb_tx_desc
+      has to be small enough so that the PMD driver will run out of available
+      Tx descriptors and free mbufs timely. Otherwise, guest Tx vring would be
+      starved.
+
+    * Guest memory should be backended with huge pages to achieve better
+      performance. Using 1G page size is the best.
+
+      When dequeue zero copy is enabled, the guest phys address and host phys
+      address mapping has to be established. Using non-huge pages means far
+      more page segments. To make it simple, DPDK vhost does a linear search
+      of those segments, thus the fewer the segments, the quicker we will get
+      the mapping. NOTE: we may speed it by using tree searching in future.
+

 * ``rte_vhost_driver_session_start()``
 
   This function starts the vhost session loop to handle vhost messages. It
 * ``rte_vhost_driver_session_start()``
 
   This function starts the vhost session loop to handle vhost messages. It


@@ -139,35 +152,8 @@ The following is an overview of the Vhost API functions:
   default.
 
 
   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
-receive vhost open/release/IOCTL messages from the QEMU simulator.
-
-When the open call is received, the vhost driver will create a vhost device
-for the virtio device in the guest.
-
-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 the ``VHOST_NET_SET_BACKEND``
-message. The registered callback from vSwitch will be called.
-
-When the release call is made, vhost will destroy the device.
-
-Vhost-user implementation
-~~~~~~~~~~~~~~~~~~~~~~~~~
+Vhost-user Implementations
+--------------------------

 
 Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
 vhost-user implementation has two options:
 
 Vhost-user uses Unix domain sockets for passing messages. This means the DPDK
 vhost-user implementation has two options:


@@ -214,8 +200,6 @@ 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.
 
 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
-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.
 ``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.