Document internals of audio virtualization

doc.md

@@ -249,6 +249,7 @@ redirect_from:
  * [Networking in Qubes](/doc/networking/)
  * [Implementation of template sharing and updating](/doc/template-implementation/)
  * [Storage Pools](/doc/storage-pools/)
+ * [Audio virtualization](/doc/audio-virtualization/)
 
 ### Services
 

system/audio.md

@@ -0,0 +1,58 @@
+---
+layout: doc
+title: Audio virtualization
+permalink: /doc/audio-virtualization/
+---
+
+Qubes audio virtualization - internals
+======================================
+
+VMs on Qubes OS have access to virtualized audio through Pulseaudio module.
+It consists of two parts:
+
+ - `pacat-simple-vchan` running in a dom0/Audio VM (standalone application, one per VM, connected to PulseAudio daemon)
+ - `module-vchan-sink` running in a VM (loaded into PulseAudio process)
+
+Protocol
+--------
+
+The protocol between those two is designed to be as simple as possible.
+Specifically, there is no audio format negotiation, no compression etc. All the audio data is transferred as raw 44.1kHz samples in S16LE format, 2 channels.
+
+Those two parts are connected with two vchan links:
+
+1. Connection on vchan port 4713 used to transfer audio from VM to Dom0/Audio VM. There is no negotiation, headers etc - raw samples are sent over this channel.
+2. Connection on vchan port 4714 used to transfer audio from Dom0/Audio VM to VM. Similar to previous one, audio data is sent as raw samples.
+
+Additionally, the second vchan connection (on port 4714) is used in the opposite direction (VM->dom0) to send simple notifications/commands about audio state. Each such notification is a 4-bytes number in little-endian format.
+
+List of defined codes:
+
+ - `0x00010001` - VM want to receive audio input (some process is listening); prior to this message, `pacat-simple-vchan` will not send any audio samples to the VM
+ - `0x00010000` - VM do not want to receive audio input (no process is listening anymore); after this message, `pacat-simple-vchan` will not send any audio samples to the VM
+ - `0x00020000` - VM do not want to send audio output; informational for dom0, to avoid buffer under runs (may affect pulseaudio calculated delays)
+ - `0x00020001` - VM do want to send audio output
+
+pacat-simple-vchan
+------------------
+
+Dom0 (or Audio VM) part of the audio virtualization. It is responsible for transferring audio samples between pulseaudio daemon in Dom0/Audio VM (having access to actual audio hardware) and a VM playing/recording.
+For each VM, matching `pacat-simple-vchan` process is running. Each of them open a separate input and output streams to local pulseaudio, which allows to control volume of each VM separately (using `pavucontrol` tool for example).
+
+Audio input to the VM is not sent by default. It needs to be both requested by the VM part and explicitly enabled in `pacat-simple-vchan`. Mechanism to do that differs between Qubes versions.
+In Qubes before R4.1, `pacat-simple-vchan` is controlled over system D-Bus:
+
+  - destination: `org.qubesos.Audio.VMNAME` (with `VMNAME` replaces with actual VM name)
+  - object path: `/org/qubesos/audio`
+  - interface: `org.qubesos.Audio`
+  - property: `RecAllowed` (settable using `org.freedesktop.DBus.Properties` interface)
+
+In Qubes R4.1 or later, `pacat-simple-vchan` is controlled over a UNIX socket at `/var/run/qubes/audio-control.VMNAME` (with `VMNAME` replaces with actual VM name). Supported commands:
+
+  - `audio-input 1\n` - enable audio input
+  - `audio-input 0\n` - disable audio input
+
+Those commands can be send using `qubes.AudioInputEnable+VMNAME` and `qubes.AudioInputDisable+VMNAME` qrexec services respectively.
+The current status is written into QubesDB at `/audio-input/VMNAME` (with `VMNAME` replaces with actual VM name) with values either `1` or `0`. Lack of the key means the `pacat-simple-vchan` for given VM is not running.
+
+In either version, it is exposed to the user as device of class `mic`, which can be attached to a VM (for example using `qvm-device mic` command).