[9/9] doc: add vfio-user library guide

diff --git a/doc/guides/prog_guide/index.rst b/doc/guides/prog_guide/index.rst
index 45c7dec88d..f9847b1058 100644
--- a/doc/guides/prog_guide/index.rst
+++ b/doc/guides/prog_guide/index.rst
@@ -70,3 +70,4 @@  Programmer's Guide
     lto
     profile_app
     glossary
+    vfio_user_lib
diff --git a/doc/guides/prog_guide/vfio_user_lib.rst b/doc/guides/prog_guide/vfio_user_lib.rst
new file mode 100644
index 0000000000..6daec4d8e5
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+++ b/doc/guides/prog_guide/vfio_user_lib.rst
@@ -0,0 +1,215 @@ 
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2020 Intel Corporation.
+
+Vfio User Library
+=============
+
+The vfio-user library implements the vfio-user protocol, which is a protocol
+that allows an I/O device to be emulated in a separate process outside of a
+Virtual Machine Monitor (VMM). The protocol has a client/server model, in which
+the server emulates the device and the client (e.g., VMM) consumes the device.
+Vfio-user library uses the device model of Linux kernel VFIO and core concepts
+defined in its API. The main difference between kernel VFIO and vfio-user is
+that the device consumer uses messages over a UNIX domain socket instead of
+system calls in vfio-user.
+
+The vfio-user library is used to construct and consume emulated devices. The
+server side implementation is mainly for construction of devices and the client
+side implementation is mainly for consumption and manipulation of devices. You
+use server APIs mainly for two things: provide the device information (e.g.,
+region/irq information) to vfio-user library and acquire the configuration
+(e.g., DMA table) from client. To construct a device, you could only focus on
+the device abstraction that vfio-user library defines rather than how the
+server side communicated with client. You use client APIs mainly for acquiring
+the device information and configuring the device. The client API usage is
+almost the same as the kernel VFIO ioctl.
+
+
+Vfio User Server API Overview
+------------------
+
+The following is an overview of key Vfio User Server API functions. You will
+know how to build an emulated device with this overview.
+
+There are mainly four steps of using Vfio User API to build your device:
+
+1. Register
+
+This step includes one API in Vfio User.
+
+* ``rte_vfio_user_register(sock_addr, notify_ops)``
+
+  This function registers a session to communicate with vfio-user client. A
+  session maps to one device so that a device instance will be created upon
+  registration.
+
+  ``sock_addr`` specifies the Unix domain socket file path and is the identity
+  of the session.
+
+  ``notify_ops`` is the a set of callbacks for vfio-user library to notify
+  emulated device. Currently, there are five callbacks:
+
+  - ``new_device``
+    This callback is invoked when the device is configured and becomes ready.
+    The dev_id is for vfio-user library to identify one uniqueue device.
+
+  - ``destroy_device``
+    This callback is invoked when the device is destroyed. In most cases, it
+    means the client is disconnected from the server.
+
+  - ``update_status``
+    This callback is invoked when the device configuration is updated (e.g.,
+    DMA table/IRQ update)
+
+  - ``lock_dp``
+    This callback is invoked when data path needs to be locked or unlocked.
+
+  - ``reset_device``
+    This callback is invoked when the emulated device need reset.
+
+2. Set device information
+
+This step includes three APIs in Vfio User.
+
+* ``rte_vfio_user_set_dev_info(sock_addr, dev_info)``
+
+  This function sets the device information to vfio-user library. The device
+  information is defined in Linux VFIO which mainly consists of device type
+  and the number of vfio regions and IRQs.
+
+* ``rte_vfio_user_set_reg_info(sock_addr, reg)``
+
+  This function sets the vfio region information to vfio-user library. Regions
+  should be created before using this API. The information mainly includes the
+  process virtual address, size, file descriptor, attibutes and capabilities of
+  regions.
+
+* ``rte_vfio_user_set_irq_info(sock_addr, irq)``
+
+  This function sets the IRQ information to vfio-user library. The information
+  includes how many IRQ type the device supports (e.g., MSI/MSI-X) and the IRQ
+  count of each type.
+
+3. Start
+
+This step includes one API in Vfio User.
+
+* ``rte_vfio_user_start(sock_addr)``
+
+  This function starts the registered session with vfio-user client. This means
+  a control thread will start to listen and handle messages sent from the client.
+  Note that only one thread is created for all vfio-user based devices.
+
+  ``sock_addr`` specifies the Unix domain socket file path and is the identity
+  of the session.
+
+4. Get device configuration
+
+This step includes two APIs in Vfio User. Both APIs should be called when the
+device is ready could be updated anytime. A simple usage of both APIs is using
+them in new_device and update_status callbacks.
+
+* ``rte_vfio_user_get_mem_table(dev_id)``
+
+  This function gets the DMA memory table from vfio-user library. The memory
+  table entry has the information of guest physical address, process virtual
+  address, size and file descriptor. Emulated devices could use the memory
+  table to perform DMA read/write on guest memory.
+
+  ``dev_id`` specifies the device ID.
+
+* ``rte_vfio_user_get_irq(dev_id, index, count, fds)``
+
+  This function gets the IRQ's eventfd from vfio-user library. In vfio-user
+  library, an efficient way to send interrupts is using eventfds. The eventfd
+  should be sent from client. Emulated devices could only call eventfd_write
+  to trigger interrupts.
+
+  ``dev_id`` specifies the device ID.
+
+  ``index`` specifies the interrupt type. The mapping of interrupt index and
+  type is defined by emulated device.
+
+  ``count`` specifies the interrupt count.
+
+  ``fds`` is for saving the file descriptors.
+
+
+Vfio User Client API Overview
+------------------
+
+The following is an overview of key Vfio User Client API functions. You will
+know how to use an emulated device with this overview.
+
+There are mainly three steps of using Vfio User Client API to consume the
+device:
+
+1. Attach
+
+This step includes one API in Vfio User.
+
+* ``rte_vfio_user_attach_dev(sock_addr)``
+
+  This function attaches to an emulated device. After the function call
+  success, it is viable to acquire device information and configure the device
+
+  ``sock_addr`` specifies the Unix domain socket file path and is the identity
+  of the session/device.
+
+2. Get device information
+
+This step includes three APIs in Vfio User.
+
+* ``rte_vfio_user_get_dev_info(dev_id, info)``
+
+  This function gets the device information of the emulated device on the other
+  side of socket. The device information is defined in Linux VFIO which mainly
+  consists of device type and the number of vfio regions and IRQs.
+
+  ``dev_id`` specifies the identity of the device.
+
+  ``info`` specifies the information of the device.
+
+* ``rte_vfio_user_get_reg_info(dev_id, info, fd)``
+
+  This function gets the region information of the emulated device on the other
+  side of socket. The region information is defined in Linux VFIO which mainly
+  consists of region size, index and capabilities.
+
+  ``info`` specifies the information of the region.
+
+  ``fd`` specifies the file descriptor of the region.
+
+* ``rte_vfio_user_get_irq_info(dev_id, irq)``
+
+  This function sets the IRQ information to vfio-user library. The IRQ
+  information includes IRQ count and index.
+
+  ``info`` specifies the information of the IRQ.
+
+3. Configure the device
+
+This step includes three APIs in Vfio User.
+
+* ``rte_vfio_user_dma_map(dev_id, mem, fds, num)``
+
+  This function maps DMA memory regions for the emulated device.
+
+  ``mem`` specifies the information of DMA memory regions.
+
+  ``fds`` specifies the file descriptors of the DMA memory regions.
+
+  ``num`` specifies the number of the DMA memory regions.
+
+* ``rte_vfio_user_dma_map(dev_id, mem, num)``
+
+  This function unmaps DMA memory regions for the emulated device.
+
+* ``rte_vfio_user_set_irqs(dev_id, set)``
+
+  This function configure the interrupts for the emulated device.
+
+  ``set`` specifies the configuration of interrupts.
+
+After the above three steps are done, users can easily use the emulated device
+(e.g., do I/O operations).
\ No newline at end of file
diff --git a/doc/guides/rel_notes/release_21_02.rst b/doc/guides/rel_notes/release_21_02.rst
index 638f98168b..6fbb6e8c39 100644
--- a/doc/guides/rel_notes/release_21_02.rst
+++ b/doc/guides/rel_notes/release_21_02.rst
@@ -55,6 +55,17 @@  New Features
      Also, make sure to start the actual text at the margin.
      =======================================================
 
+* **Added vfio-user Library.**
+
+  Added an experimental library ``librte_vfio_user`` to provide device
+  emulation support.
+
+  The library is an implementation of vfio-user protocol. It includes two main
+  parts: client and server. Server implementation is for device provider to
+  abstract its device. Client implementation is for device consumer to
+  manipulate the emulated device.
+
+  See :doc:`../prog_guide/vfio_user_lib` for more information.
 
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