docs: Add Documentation for Mediated devices

/*

 * 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

	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.