= VD Interfaces
:revnumber: v1.0
:revdate: 01.01.2009
:revremark: Draft 1

Copyright (C) 2009 Red Hat, Inc. +
Licensed under a Creative Commons Attribution-Share Alike 3.0 +
United States License (see
http://creativecommons.org/licenses/by-sa/3.0/us/legalcode). +

:toc:

== Introduction

Virtual Device Interfaces (VDI) provide a standard way to publish interfaces of
virtual devices by a software component. This enables other software components
to interact with these devices. This also enables other software components to
interact with these devices. Going forward, the first component will be called
the back-end and the second component will be called the front-end. An example
for using Virtual Device Interfaces is as part of a virtual machine system,
where the back-end will be the hardware emulation layer. The back-end will
expose interfaces like display port, mouse input etc. The front-end will plug
into the display output and will render its output according to it's specific
implementation. The front-end will also plug into the mouse input and send
mouse events to be processed by the back-end. In addition many other interface
types can be exposed by the back-end. Another example of back-end is a remote
display system in a physical machine environment. Here, the back-and is
implemented using known techniques for interacting with the native OS for the
purpose of receiving display updates and pushing inputs. The back-end exposes
interfaces like display output, mouse input etc. The front-end can be exactly
the same as in the previous example.

By using VDI one back-end can use many types of front-ends without any special
code modification. It is also possible for the back-end to dynamically switch
front-ends, and improve back-end usability and flexibility. The use of
front-ends by many back-ends allows for a better sharing of development,
maintenance, and overall product quality.

== Basic operation

Back-end to front-end interaction is initiated by back-end. Back-end uses
VDI_init symbol to pass its Core interface to the front-end. In addition to
core interface, back-end also passes options argument to the front-end. Options
argument is a string holding initialization argument specific to front-end
implementation. Core interface, like every other interface in VDI, is
represented as a data structure containing data members and member functions.
Every interface starts with a common base structure “VDInterface”. The common
structure contains information for identifying the type of the interface and
the instance within the type group (i.e. The actual unique is {type, id}). Core
interface provides basic functionality for attaching with other interfaces that
the back-end implements. Core interface provides methods for receiving
interface change events (i.e. interface added event and removing interface
event) and for enumerating registers. Using these services, the front-end can
find and interact with other interfaces that back-end publishes. The front-end
uses VDI type for knowing what functionality the interface provides and the
means to interact with the interface (i.e interface specific logic function and
data members).

== Front-end public symbols

VDI defines a minimal set of external symbols for enabling back-end to initiate
with the front-end. The back-end can load front-end and retrieve front-end
external symbols dynamically at run-time using native platform services.
Alternatively, the front-end static or shared library can be linked to the
back-end during back-end build process.

[source,c]
----
int VDI_init(const char *args, CoreInterface *core)
----

Front-and initialization function. The first parameter is options argument, the
content of  which depends on the specific front-end. The second parameter is
core interface that provides the basic communication channel between front-end
and back-end, core interface will be explained later on. VDI_init return value
is VDI_ERROR_?

[source,c]
----
const char **VDI_get_usage()
----

This function return array of strings in human readable form for displaying the
available options argument that can be pass in args parameter of VDI_init call.

== Front-end termination

No explicit symbol is defined for front-end termination. Removing core
interface by back-end implicitly instructs front end to terminate itself.

== Common Definition

[source,c]
----
VDI_ERROR_OK = 0
VDI_ERROR_ERROR = -1
VM_INTERFACE_VERSION = 2 /*Current VDI version, front-end an back-end must have
the same VDI version for proper operation. */
----
----
VDIObjectRef – opaque object reference that passes between back-end and
front-end
----
[source,c]
----
VDI_INVALID_OBJECT_REF = 0
----
== Base structure of VD interface.

The following is definition of VD Interface, all VDI interfaces are derived
from VD Interface.

[source,c]
----
UINT32 version; /* VDI version must be VM_INTERFACE_VERSION */
CONST  CHAR *type; /* type of the interface */
UNSIGNED INT id; /* unique id of the interface */
CONST  CHAR *description; /* interface human readable interface description*
UINT32 major_version; /* interface major version. Front-end and back-end
interfaces having different  major_version are not compatible. */
UINT32 minor_version; /* interface minor version. Minor version increments on
changes that add functionality but do not break compatible and will be reset on
major version  increment. */
----

== Core Interface

Core interface is the main interface for front-end to back-end communication.
It is not allowed to have more the one interface with this type. Removal of
Core interface by the back-end is explicit front-end termination.

. VDInterface members value
+
----
type = “core”
major_version = 1
minor_version = 0
id = 0
----
+
. Types
+
[source,c]
----
VDI_CHANGE_NEW = 0
VDI_CHANGE_REMOVING = 1
----
+
[source,c]
----
void (*vdi_interface_change_notifier_t)(void *opaque, VDInterface *interface,
VDInterfaceChangeType change)
----
+
Callback for receiving notification event on adding and removing VDI
interfaces.
+
* opaque – back-end passes front-end's opaque value that was passed during
notifier  registration.
* interface - the relevant interface
* change – type of change VDI_CHANGE_NEW or VDI_CHANGE_REMOVING

+
[source,c]
----
void (*timer_callback_t)(void *opaque)
----
+
Callback for receiving timer event.
+
* opaque – back-end passes front-end's opaque value that was passed during
timer registration.

+
----
VDI_LOG_LEVEL_ERROR = 1
VDI_LOG_LEVEL_WARN = 2
VDI_LOG_LEVEL_INFO = 3
----
+
[source,c]
----
VDInterface specific extension
VDInterface *(* next)(CoreInterface *core, VDInterface *prev)
----
+
Get next interface – return interface following prev. If prev is NULL  return
first interface. If next return NULL then prev is the last interface. The
function is used for interfaces discovery.
+
[source,c]
----
VDIObjectRef (*register_change_notifier)(CoreInterface *core, void *opaque,
vdi_interface_change_notifier_t in_notifier)
----
+
Register in_notifier to be called on interface change events. opaque will be
pushed back by back-end in notify call. On success the function return notifier
object ref otherwise VDI_INVALID_OBJECT_REF is returned.
+
[source,c]
----
void (*unregister_change_notifiers)(CoreInterface *core, VDObjectRef notifier)
----
+
Unregister interface change notifier hat was registered using
register_change_notifiers.
+
[source,c]
----
VDIObjectRef (*create_timer)(CoreInterface *core, timer_callback_t callback,
void* opaque)
----
+
Create timer object. The timer handler will call callback on time expiration.
opaque will be pushed back by back-end in callback call. On success the
function return timer object ref otherwise VDI_INVALID_OBJECT_REF is returned
+
[source,c]
----
void (*arm_timer)(CoreInterface *core, VDIObjectRef timer, uint32_t ms)
----
+
Schedule timer object to fire it's callback after ms. Timer is VDI ref object
returned by create_timer and ms is time in millisecond.
+
[source,c]
----
void (*disarm_timer)(CoreInterface *core, VDIObjectRef timer)
----
+
Cancel previous arm_timer scheduling. Timer is VDI ref object returned by
create_timer
+
[source,c]
----
void (*destroy_timer)(CoreInterface *core, VDIObjectRef timer)
----
+
Destroy timer object. Timer is vdi ref object returned by create_timer.
+
[source,c]
----
int (*set_file_handlers)(CoreInterface *core, int fd, void (*on_read)(void *),
void (*on_write)(void *), void *opaque)
----
+
Request notification on file descriptor for non blocking operation. Fd is the
target file descriptor. Back-end will call on_read, if not NULL, on read events
passing  opaque as first argument. Back-end will call on_write, if not NULL, on
write events passing  opaque as first argument. On success VDI_ERROR_OK is
return.
+
[source,c]
----
void (*term_printf)(CoreInterface *core, const char* format, ...)
----
+
Send message to back-end interactive terminal if exist. format is printf
format,  following format are variable arguments list that are format
arguments.
+
[source,c]
----
void (*log)(CoreInterface *core, LogLevel level, const char* component, const
char* format, ...)
----
+
Send message to be logged using back-end logging service. Level is one of
VDI_LOG_LEVEL_?.  Component is component name that will be attached as prefix
to the message. Format is printf format, following format are variable
arguments list that are format arguments.
Callback

== Other interfaces

Core interface is the only interface that is an integral part of VDI
specifications. Any other interface can be defined independently of this
specification. It's required that both the back-and the front-end know specific
interface definitions for proper operation. The only restriction for defining
new interface is to ensure the uniqueness of VD Interface type. The following
interfaces are known VD interfaces that discussed here as a reference for
better understanding of the specification.

== Keyboard  interface

Keyboard interface exposes standard keyboard functionally. It enables pushing
scan-code set 1 codes and getting keyboard LED information by query or change
notifications mechanism.

. VDInterface members value
+
[source,c]
----
type = “keyboard”
major_version = 1
minor_version = 0
id = keyboard instance id
----
+
. Types
+
[source,c]
----
VDI_KEYBOARD_LED_SCROLL_LOCK 0
VDI_KEYBOARD_LED_NUM_LOCK 1
VDI_KEYBOARD_LED_CAPS_LOCK 2
----
+
[source,c]
----
void (*keyboard_leads_notifier_t)(void *opaque, uint8_t leds)
----
+
Callback for receiving notification event on keyboard LED changes.
+
* opaque – back-end passed front-end's opaque value that was passed during
notifier  registration.
* leds – current LED state mask. LED X is on if bit VDI_KEYBOARD_LED_X is set.

+
. VDInterface specific extension
+
[source,c]
----
void (*push_scan_freg)(KeyboardInterface *keyboard, uint8_t frag)
----
+
Push  scan-code code fragment to the keyboard.  Frag is part of scan code.
For example calling:
+
[source,c]
----
push_scan_freg(0xe0);
push_scan_freg(0x1d);
----
+
will result with Right CTRL.
+
[source,c]
----
uint8_t (*get_leds)(KeyboardInterface *keyboard)
----
+
Get current keyboard lads mask. Return lads mask
+
[source,c]
----
VDIObjectRef (*register_leds_notifier)(KeyboardInterface *keyboard,
keyboard_leads_notifier_t notifier, void *opaque)
----
+
Register for LED notification. Back-end will call notifier on every keyboard
LED change. Opaque will be pushed back as notifier argument. On success, the
function return notifier object ref otherwise VDI_INVALID_OBJECT_REF is
returned.
+
[source,c]
----
void (*unregister_leds_notifier)(KeyboardInterface *keyboard, VDObjectRef
notifier)
----
+
Unregister  LED change notifier that was registered using
register_leds_notifier.

== Mouse  interface

Mouse interface for pushing mouse  motion and mouse buttons state events.

. VDInterface members value
+
[source,c]
----
type = “mouse”
major_version = 1
minor_version = 0
id = mouse instance id
----
+
. Types
+
[source,c]
----
VDI_MOUSE_BUTTON_LEFT 0
VDI_MOUSE_BUTTON_RIGHT 1
VDI_MOUSE_BUTTON_MIDEL 2
----
+
. VDInterface specific extension
+
[source,c]
----
void (*motion)(MouseInterface* mouse, int dx, int dy, int dz, uint32_t
buttons_state)
----
+
Push mouse motion and buttons state. dx and dy are mouse motion in pixels in x
and y axis, positive dx is rightward motion and positive dy is downward motion.
dz is scroll wheel ticks, positive value is scroll down. buttons_state is
buttons state mask. Button X is press if bit VDI_MOUSE_BUTTON_X is set.
+
[source,c]
----
void (*buttons)(MouseInterface* mouse, uint32_t buttons_state)
----
+
Push mouse buttons state. buttons_state is the same as in the motion call

== Tablet  interface

Tablet interface for supporting absolute mouse pointing.

. Interface members value
+
[source,c]
----
type = “tablet”
major_version = 1
minor_version = 0
id = tablet instance id
----
+
. Types
+
[source,c]
----
VDI_TABLET_BUTTON_LEFT 0
VDI_TABLET_BUTTON_RIGHT 1
VDI_TABLET_BUTTON_MIDEL 2
----
+
. VDInterface specific extension
+
[source,c]
----
void (*set_logical_size)(TabletInterface* tablet, int width, int height)
----
+
Set logical dimension of the tablet. The logical dimension will be used by the
tablet device for mapping position to local coordinate system.
+
[source,c]
----
void (*position)(TabletInterface* tablet, int x, int y, uint32_t buttons_state)
----
+
Push mouse position and mouse button state.  {x, y} position is relative to the
upper left corner. Positive value on x axis advances rightward and positive
value on y axis advances downward. buttons_state is buttons state mask. Button
X is press if bit VDI_TABLET_BUTTON_X is set.
+
[source,c]
----
void (*wheel)(TabletInterface* tablet, int wheel_motion, uint32_t
buttons_state)
----
+
Push scroll wheel motion and buttons state. wheel_motion is scroll sicks.
positive wheel_motion is for scroll down. buttons_state is as in “position”
+
[source,c]
----
void (*buttons)(TabletInterface* tablet, uint32_t buttons_state)
----
+
Push buttons state. buttons_state is as in “position”

== Playback  interface

Playback interface for receiving audio stream for playback. Playback use fix
audio configuration,  channels = 2,  frequency = 44100 and channel sample
format signed 16bit.

. VDInterface members value
+
[source,c]
----
type = “playback”
major_version = 1
minor_version = 0
id = playback instance id
----
+
. PlaybackPlug
+
[source,c]
----
uint32_t major_version
----
+
plug major version. Changing  major version breaks compatibility.
+
[source,c]
----
uint32_t minor_version
----
+
interface minor version. Minor version increments on changes that add
functionality but do not break compatible and will be reset on major version
increment.
+
[source,c]
----
void (*start)(PlaybackPlug *plug)
----
+
back-end will call this callback while staring audio playback
+
[source,c]
----
void (*stop)(PlaybackPlug *plug)
----
+
back-end will call this callback while stopping audio playback
+
[source,c]
----
void (*get_frame)(PlaybackPlug *plug, uint32_t **frame, uint32_t *samples)
----
+
back-end will call this callback for getting audio frame. On return *frame will
point to audio frame capable of holding *samples samples (i.e frame size is
samples * 4). The frame will be pushed back filled with audio samples by the
back-end. If no frame is available *frame will be NULL.
+
[source,c]
----
void (*put_frame)(PlaybackPlug *plug, uint32_t *frame)
----
+
back-end will call this callback to deliver audio frame for playback. Frame is
one that was returns by get_frame. This call also moves frame ownership back to
the front-end.

. VDInterface specific extension
+
[source,c]
----
VDIObjectRef (*plug)(PlaybackInterface *playback, PlaybackPlug* plug,  int
*active)
----
+
Attach playback plug to playback interface instance. Plug is the plug to
attach. On return, active will be 0 if playback is stopped and 1 if playback is
running. On success, the function return object ref otherwise
VDI_INVALID_OBJECT_REF is returned.
+
[source,c]
----
void (*unplug)(PlaybackInterface *playback, VDIObjectRef)
----
+
Unplug  playback that was plugged using plug.

== Record  interface

Record interface for pushing audio capture to back-end. Record use fix audio
configuration, channels = 2,  frequency = 44100 and channel sample format
signed 16bit.

. VDInterface members value
+
[source,c]
----
type =  “record”
major_version = 1
minor_version = 0
id = record instance id
----

. RecordPlug
+
[source,c]
----
uint32_t minor_version
----
+
plug major version. Changing  major version breaks compatibility.
+
[source,c]
----
uint32_t major_version
----
+
interface minor version. Minor version increments on changes that add
functionality but do not break compatible and will be reset on major version
increment.
+
[source,c]
----
void (*start)(RecordPlug *plug)
----
+
back-end will call this callback for staring audio capture
+
[source,c]
----
void (*stop)(RecordPlug *plug)
----
+
back-end will call this callback to stop audio capture
+
[source,c]
----
uint32_t (*read)(RecordPlug *plug, uint32_t num_samples, uint32_t *samples)
----
+
back-end will call this callback to receive audio capture samples.  num_samples
is the requested number samples. Samples is buffer for receiving samples (i.e
samples size is num_samples * 4). Read return number of samples returned.

. VDInterface specific extension
+
[source,c]
----
VDIObjectRef (*plug)(RecordInterface *recorder, RecordPlug* plug, int *active)
----
+
Attach record plug to interface instance. Plug is the plug to attach. On
return, active will be 0 if record is stopped and 1 if record is running. On
success, the function return object ref otherwise VDI_INVALID_OBJECT_REF is
returned.
+
[source,c]
----
void (*unplug)(RecordInterface *recorder, VDIObjectRef)
----
+
Unplug  record plug that was plugged using plug.

== Additional registered interfaces

* QXL interface "qxl" – QXL interface enable front-end to plug into QXL display
device in order to receive QXL and to render it's output locally or remotely.
* VDI port interface "vdi_port" – interface for plugging, reading and writing
to vdi_port. vdi_port is used for communication with an agent residing on gust
machine.
* Migration  interface "migration" -  interface for integrating with back-end
migration process. This interface enable registering for pre and post migration
hooks.
* Qemu terminal  interface "qemu_terminal" – interface for integrating with
QEMU style terminal.