THE ANDROID "QEMUD" MULTIPLEXING DAEMON I. Overview: ------------ The Android system image includes a small daemon program named "qemud" which is started at boot time. Its purpose is to provide a multiplexing communication channel between the emulated system and the emulator program itself. Another way to support communication between the emulated system and the emulator program is using qemu pipes (see ANDROID-QEMU-PIPE.TXT for details on qemu pipes). Its goal is to allow certain parts of the system to talk directly to the emulator without requiring special kernel support; this simplifies a lot of things since it does *not* require: - writing/configuring a specific kernel driver - writing the corresponding hardware emulation code in hw/android/goldfish/*.c - dealing with device allocation and permission issues in the emulated system The emulator provides 'services' to various parts of the emulated system. Each service is identified by a name and serves a specific purpose. For example: "gsm" Used to communicate with the emulated GSM modem with AT commands. "gps" Used to receive NMEA sentences broadcasted from the emulated GPS device. "sensors" Used to list the number of emulated sensors, as well as enable/disable reception of specific sensor events. "control" Used to control misc. simple emulated hardware devices (e.g. vibrator, leds, LCD backlight, etc...) II. Implementation: ------------------- Since the "cupcake" platform, this works as follows: - A 'qemud client' is any part of the emulated system that wants to talk to the emulator. It does so by: - connecting to the /dev/socket/qemud Unix domain socket - sending the service name through the socket - receives two bytes of data, which will be "OK" in case of success, or "KO" in case of failure. After an OK, the same connection can be used to talk directly to the corresponding service. - The /dev/socket/qemud Unix socket is created by init and owned by the 'qemud' daemon started at boot by /system/etc/init.goldfish.rc The daemon also opens an emulated serial port (e.g. /dev/ttyS1) and will pass all messages between clients and emulator services. Thus, everything looks like the following: emulator <==serial==> qemud <---> /dev/socket/qemud <-+--> client1 | +--> client2 A very simple multiplexing protocol is used on the serial connection: offset size description 0 2 2-char hex string giving the destination or source channel 2 4 4-char hex string giving the payload size 6 n the message payload Where each client gets a 'channel' number allocated by the daemon at connection time. Note that packets larger than 65535 bytes cannot be sent directly through the qemud channel. This is intentional; for large data communication, the client and service should use a fragmentation convention that deals with this. Zero-sized packets are silently discard by qemud and the emulator and should normally not appear on the serial port. Channel 0 is reserved for control messages between the daemon and the emulator. These are the following: - When a client connects to /dev/socket/qemud and sends a service name to the daemon, the later sends to the emulator: connect:: where is the service name, and is a 2-hexchar string giving the allocated channel index for the client. - The emulator can respond in case of success with: ok:connect: or, in case of failure, with: ok:connect:: where is a liberal string giving the reason for failure. It is never sent to clients (which will only receive a "KO") and is used strictly for debugging purposes. - After a succesful connect, all messages between the client and the corresponding emulator service will be passed through the corresponding numbered channel. But if the client disconnects from the socket, the daemon will send through channel 0 this message to the emulator: disconnect: - If an emulator service decides, for some reason, to disconnect a client, the emulator will send to the daemon (on channel 0): disconnect: The daemon deals with this gracefully (e.g. it will wait that the client has read all buffered data in the daemon before closing the socket, to avoid packet loss). - Any other command sent from the daemon to the emulator will result in the following answer: ko:bad command - Which exact serial port to open is determined by the emulator at startup and is passed to the system as a kernel parameter, e.g.: android.qemud=ttyS1 - The code to support services and their clients in the emulator is located in android/hw-qemud.c. This code is heavily commented. The daemon's source is in $ROOT/development/emulator/qemud/qemud.c The header in $ROOT/hardware/libhardware/include/hardware/qemud.h can be used by clients to ease connecting and talking to QEMUD-based services. This is used by $ROOT/developement/emulator/sensors/sensors_qemu.c which implements emulator-specific sensor support in the system by talking to the "sensors" service provided by the emulator (if available). Code in $ROOT/hardware/libhardware_legacy also uses QEMUD-based services. - Certain services also implement a simple framing protocol when exchanging messages with their clients. The framing happens *after* serial port multiplexing and looks like: offset size description 0 4 4-char hex string giving the payload size 4 n the message payload This is needed because the framing protocol used on the serial port is not preserved when talking to clients through /dev/socket/qemud. Certain services do not need it at all (GSM, GPS) so it is optional and must be used depending on which service you talk to by clients. - QEMU pipe communication model works similarly to the serial port multiplexing, but also has some differences as far as connecting client with the service is concerned: emulator <-+--> /dev/qemu_pipe/qemud:srv1 <---> client1 | +--> /dev/qemu_pipe/qemud:srv2 <---> client2 In the pipe model each client gets connected to the emulator through a unique handle to /dev/qemu_pipe (a "pipe"), so there is no need for multiplexing the channels. III. Legacy 'qemud': -------------------- The system images provided by the 1.0 and 1.1 releases of the Android SDK implement an older variant of the qemud daemon that uses a slightly different protocol to communicate with the emulator. This is documented here since this explains some subtleties in the implementation code of android/hw-qemud.c The old scheme also used a serial port to allow the daemon and the emulator to communicate. However, the multiplexing protocol swaps the position of 'channel' and 'length' in the header: offset size description 0 4 4-char hex string giving the payload size 4 2 2-char hex string giving the destination or source channel 6 n the message payload Several other differences, best illustrated by the following graphics: emulator <==serial==> qemud <-+--> /dev/socket/qemud_gsm <--> GSM client | +--> /dev/socket/qemud_gps <--> GPS client | +--> /dev/socket/qemud_control <--> client(s) Now, for the details: - instead of a single /dev/socket/qemud, init created several Unix domain sockets, one per service: /dev/socket/qemud_gsm /dev/socket/qemud_gps /dev/socket/qemud_control note that there is no "sensors" socket in 1.0 and 1.1 - the daemon created a de-facto numbered channel for each one of these services, even if no client did connect to it (only one client could connect to a given service at a time). - at startup, the emulator does query the channel numbers of all services it implements, e.g. it would send *to* the daemon on channel 0: connect: where can be one of "gsm", "gps" or "control" (Note that on the current implementation, the daemon is sending connection messages to the emulator instead). - the daemon would respond with either: ok:connect:: where would be the service name, and a 4-hexchar channel number (NOTE: 4 chars, not 2). Or with: ko:connect:bad name This old scheme was simpler to implement in both the daemon and the emulator but lacked a lot of flexibility: - adding a new service required to modify /system/etc/init.goldfish.rc as well as the daemon source file (which contained a hard-coded list of sockets to listen to for client connections). - only one client could be connected to a given service at a time, except for the GPS special case which was a unidirectionnal broadcast by convention. The current implementation moves any service-specific code to the emulator, only uses a single socket and allows concurrent clients for a all services. IV. State snapshots: -------------------- Support for snapshots relies on the symmetric qemud_*_save and qemud_*_load functions which save the state of the various Qemud* structs defined in android/hw-qemud.c. The high-level process is as follows. When a snapshot is made, the names and configurations of all services are saved. Services can register a custom callback, which is invoked at this point to allow saving of service-specific state. Next, clients are saved following the same pattern. We save the channel id and the name of service they are registered to, then invoke a client-specific callback. When a snapshot is restored, the first step is to check whether all services that were present when the snapshot was made are available. There is currently no functionality to start start missing services, so loading fails if a service is not present. If all services are present, callbacks are used to restore service-specific state. Next, all active clients are shut down. Information from the snapshot is used to start new clients for the services and channels as they were when the snapshot was made. This completes the restore process.