docs: Add Documentation for Mediated devices

Add file Documentation/vfio-mediated-device.txt that include details of
mediated device framework.
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Neo Jia <cjia@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Documentation/vfio-mediated-device.txt

+/*
+ * VFIO Mediated devices
+ *
+ * Copyright (c) 2016, NVIDIA CORPORATION. All rights reserved.
+ *     Author: Neo Jia <cjia@nvidia.com>
+ *             Kirti Wankhede <kwankhede@nvidia.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+Virtual Function I/O (VFIO) Mediated devices[1]
+===============================================
+
+The number of use cases for virtualizing DMA devices that do not have built-in
+SR_IOV capability is increasing. Previously, to virtualize such devices,
+developers had to create their own management interfaces and APIs, and then
+integrate them with user space software. To simplify integration with user space
+software, we have identified common requirements and a unified management
+interface for such devices.
+
+The VFIO driver framework provides unified APIs for direct device access. It is
+an IOMMU/device-agnostic framework for exposing direct device access to user
+space in a secure, IOMMU-protected environment. This framework is used for
+multiple devices, such as GPUs, network adapters, and compute accelerators. With
+direct device access, virtual machines or user space applications have direct
+access to the physical device. This framework is reused for mediated devices.
+
+The mediated core driver provides a common interface for mediated device
+management that can be used by drivers of different devices. This module
+provides a generic interface to perform these operations:
+
+* Create and destroy a mediated device
+* Add a mediated device to and remove it from a mediated bus driver
+* Add a mediated device to and remove it from an IOMMU group
+
+The mediated core driver also provides an interface to register a bus driver.
+For example, the mediated VFIO mdev driver is designed for mediated devices and
+supports VFIO APIs. The mediated bus driver adds a mediated device to and
+removes it from a VFIO group.
+
+The following high-level block diagram shows the main components and interfaces
+in the VFIO mediated driver framework. The diagram shows NVIDIA, Intel, and IBM
+devices as examples, as these devices are the first devices to use this module.
+
+     +---------------+
+     |               |
+     | +-----------+ |  mdev_register_driver() +--------------+
+     | |           | +<------------------------+              |
+     | |  mdev     | |                         |              |
+     | |  bus      | +------------------------>+ vfio_mdev.ko |<-> VFIO user
+     | |  driver   | |     probe()/remove()    |              |    APIs
+     | |           | |                         +--------------+
+     | +-----------+ |
+     |               |
+     |  MDEV CORE    |
+     |   MODULE      |
+     |   mdev.ko     |
+     | +-----------+ |  mdev_register_device() +--------------+
+     | |           | +<------------------------+              |
+     | |           | |                         |  nvidia.ko   |<-> physical
+     | |           | +------------------------>+              |    device
+     | |           | |        callbacks        +--------------+
+     | | Physical  | |
+     | |  device   | |  mdev_register_device() +--------------+
+     | | interface | |<------------------------+              |
+     | |           | |                         |  i915.ko     |<-> physical
+     | |           | +------------------------>+              |    device
+     | |           | |        callbacks        +--------------+
+     | |           | |
+     | |           | |  mdev_register_device() +--------------+
+     | |           | +<------------------------+              |
+     | |           | |                         | ccw_device.ko|<-> physical
+     | |           | +------------------------>+              |    device
+     | |           | |        callbacks        +--------------+
+     | +-----------+ |
+     +---------------+
+
+
+Registration Interfaces
+=======================
+
+The mediated core driver provides the following types of registration
+interfaces:
+
+* Registration interface for a mediated bus driver
+* Physical device driver interface
+
+Registration Interface for a Mediated Bus Driver
+------------------------------------------------
+
+The registration interface for a mediated bus driver provides the following
+structure to represent a mediated device's driver:
+
+     /*
+      * struct mdev_driver [2] - Mediated device's driver
+      * @name: driver name
+      * @probe: called when new device created
+      * @remove: called when device removed
+      * @driver: device driver structure
+      */
+     struct mdev_driver {
+	     const char *name;
+	     int  (*probe)  (struct device *dev);
+	     void (*remove) (struct device *dev);
+	     struct device_driver    driver;
+     };
+
+A mediated bus driver for mdev should use this structure in the function calls
+to register and unregister itself with the core driver:
+
+* Register:
+
+  extern int  mdev_register_driver(struct mdev_driver *drv,
+				   struct module *owner);
+
+* Unregister:
+
+  extern void mdev_unregister_driver(struct mdev_driver *drv);
+
+The mediated bus driver is responsible for adding mediated devices to the VFIO
+group when devices are bound to the driver and removing mediated devices from
+the VFIO when devices are unbound from the driver.
+
+
+Physical Device Driver Interface
+--------------------------------
+
+The physical device driver interface provides the parent_ops[3] structure to
+define the APIs to manage work in the mediated core driver that is related to
+the physical device.
+
+The structures in the parent_ops structure are as follows:
+
+* dev_attr_groups: attributes of the parent device
+* mdev_attr_groups: attributes of the mediated device
+* supported_config: attributes to define supported configurations
+
+The functions in the parent_ops structure are as follows:
+
+* create: allocate basic resources in a driver for a mediated device
+* remove: free resources in a driver when a mediated device is destroyed
+
+The callbacks in the parent_ops structure are as follows:
+
+* open: open callback of mediated device
+* close: close callback of mediated device
+* ioctl: ioctl callback of mediated device
+* read : read emulation callback
+* write: write emulation callback
+* mmap: mmap emulation callback
+
+A driver should use the parent_ops structure in the function call to register
+itself with the mdev core driver:
+
+extern int  mdev_register_device(struct device *dev,
+                                 const struct parent_ops *ops);
+
+However, the parent_ops structure is not required in the function call that a
+driver should use to unregister itself with the mdev core driver:
+
+extern void mdev_unregister_device(struct device *dev);
+
+
+Mediated Device Management Interface Through sysfs
+==================================================
+
+The management interface through sysfs enables user space software, such as
+libvirt, to query and configure mediated devices in a hardware-agnostic fashion.
+This management interface provides flexibility to the underlying physical
+device's driver to support features such as:
+
+* Mediated device hot plug
+* Multiple mediated devices in a single virtual machine
+* Multiple mediated devices from different physical devices
+
+Links in the mdev_bus Class Directory
+-------------------------------------
+The /sys/class/mdev_bus/ directory contains links to devices that are registered
+with the mdev core driver.
+
+Directories and files under the sysfs for Each Physical Device
+--------------------------------------------------------------
+
+|- [parent physical device]
+|--- Vendor-specific-attributes [optional]
+|--- [mdev_supported_types]
+|     |--- [<type-id>]
+|     |   |--- create
+|     |   |--- name
+|     |   |--- available_instances
+|     |   |--- device_api
+|     |   |--- description
+|     |   |--- [devices]
+|     |--- [<type-id>]
+|     |   |--- create
+|     |   |--- name
+|     |   |--- available_instances
+|     |   |--- device_api
+|     |   |--- description
+|     |   |--- [devices]
+|     |--- [<type-id>]
+|          |--- create
+|          |--- name
+|          |--- available_instances
+|          |--- device_api
+|          |--- description
+|          |--- [devices]
+
+* [mdev_supported_types]
+
+  The list of currently supported mediated device types and their details.
+
+  [<type-id>], device_api, and available_instances are mandatory attributes
+  that should be provided by vendor driver.
+
+* [<type-id>]
+
+  The [<type-id>] name is created by adding the the device driver string as a
+  prefix to the string provided by the vendor driver. This format of this name
+  is as follows:
+
+	sprintf(buf, "%s-%s", dev_driver_string(parent->dev), group->name);
+
+* device_api
+
+  This attribute should show which device API is being created, for example,
+  "vfio-pci" for a PCI device.
+
+* available_instances
+
+  This attribute should show the number of devices of type <type-id> that can be
+  created.
+
+* [device]
+
+  This directory contains links to the devices of type <type-id> that have been
+created.
+
+* name
+
+  This attribute should show human readable name. This is optional attribute.
+
+* description
+
+  This attribute should show brief features/description of the type. This is
+  optional attribute.
+
+Directories and Files Under the sysfs for Each mdev Device
+----------------------------------------------------------
+
+|- [parent phy device]
+|--- [$MDEV_UUID]
+         |--- remove
+         |--- mdev_type {link to its type}
+         |--- vendor-specific-attributes [optional]
+
+* remove (write only)
+Writing '1' to the 'remove' file destroys the mdev device. The vendor driver can
+fail the remove() callback if that device is active and the vendor driver
+doesn't support hot unplug.
+
+Example:
+	# echo 1 > /sys/bus/mdev/devices/$mdev_UUID/remove
+
+Mediated device Hot plug:
+------------------------
+
+Mediated devices can be created and assigned at runtime. The procedure to hot
+plug a mediated device is the same as the procedure to hot plug a PCI device.
+
+Translation APIs for Mediated Devices
+=====================================
+
+The following APIs are provided for translating user pfn to host pfn in a VFIO
+driver:
+
+extern int vfio_pin_pages(struct device *dev, unsigned long *user_pfn,
+                          int npage, int prot, unsigned long *phys_pfn);
+
+extern int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn,
+                            int npage);
+
+These functions call back into the back-end IOMMU module by using the pin_pages
+and unpin_pages callbacks of the struct vfio_iommu_driver_ops[4]. Currently
+these callbacks are supported in the TYPE1 IOMMU module. To enable them for
+other IOMMU backend modules, such as PPC64 sPAPR module, they need to provide
+these two callback functions.
+
+References
+----------
+
+[1] See Documentation/vfio.txt for more information on VFIO.
+[2] struct mdev_driver in include/linux/mdev.h
+[3] struct parent_ops in include/linux/mdev.h
+[4] struct vfio_iommu_driver_ops in include/linux/vfio.h

drivers/vfio/mdev/Kconfig

@@ -5,6 +5,7 @@ config VFIO_MDEV
 	default n
 	help
 	  Provides a framework to virtualize devices.
+	  See Documentation/vfio-mediated-device.txt for more details.
 
 	  If you don't know what do here, say N.