0108: Emulators Frontend#

SEED-0108: Emulators Frontend

Status: Open for Comments Intent Approved Last Call Accepted Rejected

Proposal Date: 2023-06-24

CL: pwrev/158190

Author: Octavian Purdila

Facilitator: Armando Montanez


This SEED proposes a new Pigweed module that allows users to define emulator targets, start, control and interact with multiple running emulator instances, either through a command line interface or programmatically through Python APIs.


An emulator is a program that allows users to run unmodified images compiled for target on the host machine. The host is the machine that runs the Pigweed environment.

An emulated machine or board is an emulator abstraction that typically has a correspondence in the real world - a product, an evaluation board, a hardware platform.

An emulated machine can be extended / tweaked through runtime configuration options: add sensors on an i2c bus, connect a disk drive to a disk controller, etc.

An emulator may use an object model, a hierarchical arrangement of emulator objects which are emulated devices (e.g. SPI controller) or internal emulator structures.

An object can be accessed through an object path and can have properties. Device properties controls how the device is emulated (e.g. enables or disables certain features, defines memory sizes, etc.).

A channel is a communication abstraction between the emulator and host processes. Examples of channels that an emulator can expose to the host:

  • An emulated UART could be exposed on the host as a PTY or a socket.

  • A flash device could be exposed on the host as file.

  • A network device could be exposed on the host as a tun/tap interface.

  • A remote gdb interface could be exposed to the host as socket.

A monitor is a control channel that allows the user to interactively or programmatically control the emulator: pause execution, inspect the emulator internal state, connect new devices, etc.


While it is already possible to use emulators directly, there is a significant learning curve for using a specific emulator. Even for the same emulator each emulated machine (board) has its own peculiarities and it often requires tweaks to customize it to a specific project’s needs through command line options or scripts (either native emulator scripts, if supported, or through helper shell scripts).

Once started, the user is responsible for managing the emulator life-cycle, potentially for multiple instances. They also have to interact with it through various channels (monitor, debugger, serial ports) that requires some level of host resource management. Especially in the case of using multiple emulator instances manually managing host resources are burdensome.

A frequent example is the default debugger localhost:1234 port that can conflict with multiple emulator instances or with other debuggers running on the host. Another example: serials exposed over PTY have the pts number in /dev/pts/ allocated dynamically and it requires the user to retrieve it somehow.

This gets even more complex when using different operating systems where some type of host resources are not available (e.g. no PTYs on Windows) or with limited functionality (e.g. UNIX sockets are supported on Windows > Win10 but only for stream sockets and there is no Python support available yet).

Using emulators in CI is also difficult, in part because host resource management is getting more complicated due scaling (e.g. more chances of TCP port conflicts) and restrictions in the execution environment. But also because of a lack of high level APIs to control emulators and access their channels.


Add a new Pigweed module that:

  • Allows users to define emulation targets that encapsulate the emulated machine configuration, the tools configuration and the host channels configuration.

  • Provides a command line interface that manages multiple emulator instances and provides interactive access to the emulator’s host channels.

  • Provides a Python API to control emulator instances and access the emulator’s host channels.

  • Supports multiple emulators, QEMU and renode as a starting point.

  • Expose channels for gdb, monitor and user selected devices through configurable host resources, sockets and PTYs as a starting point.

The following sections will add more details about the configuration, the command line interface, the API for controlling and accessing emulators and the API for adding support for more emulators.


The emulators configuration is part of the Pigweed root configuration file (pigweed.json) and reside in the pw:pw_emu namespace.

Projects can define emulation targets in the Pigweed root configuration file and can also import predefined targets from other files. The pw_emu module provides a set of targets as examples and to promote reusability.

For example, the following top level pigweed.json configuration includes a target fragment from the pw_emu/qemu-lm3s6965evb.json file:

  "pw": {
    "pw_emu": {
      "target_files": [

pw_emu/qemu-lm3s6965evb.json defines the qemu-lm3s6965evb target that uses qemu as the emulator and lm3s6965evb as the machine, with the serial0 chardev exposed as serial0:

  "targets": {
    "qemu-lm3s6965evb": {
      "gdb": "arm-none-eabi-gdb",
      "qemu": {
        "executable": "qemu-system-arm",
        "machine": "lm3s6965evb",
        "channels": {
          "chardevs": {
            "serial0": {
              "id": "serial0"

This target emulates a stm32f405 SoC and is compatible with the lm3s6965evb-qemu Pigweed build target.

The configuration defines a serial0 channel to be the QEMU chardev with the serial0 id. The default type of the channel is used, which is TCP and which is supported by all platforms. The user can change the type by adding a type key set to the desired type (e.g. pty).

The following configuration fragment defines a target that uses renode:

  "targets": {
    "renode-stm32f4_discovery": {
      "gdb": "arm-none-eabi-gdb",
      "renode": {
        "executable": "renode",
        "machine": "platforms/boards/stm32f4_discovery-kit.repl",
        "channels": {
          "terminals": {
            "serial0": {
              "device-path": "sysbus.uart0",
              "type": "pty"

Note that machine is used to identify which renode script to use to load the plaform description from and terminals to define which UART devices to expose to the host. Also note the serial0 channel is set to be exposed as a PTY on the host.

The following channel types are currently supported:

  • pty: supported on Mac and Linux; renode only supports PTYs for terminals channels.

  • tcp: supported on all platforms and for all channels; it is also the default type if no channel type is configured.

The channel configuration can be set at multiple levels: emulator, target, or specific channel. The channel configuration takes precedence, then the target channel configuration then the emulator channel configuration.

The following expressions are replaced in the configuration strings:

  • $pw_emu_wdir{relative-path}: replaces statement with an absolute path by concatenating the emulator’s working directory with the given relative path.

  • $pw_emu_channel_port{channel-name}: replaces the statement with the port number for the given channel name; the channel type should be tcp.

  • $pw_emu_channel_host{channel-name}: replaces the statement with the host for the given channel name; the channel type should be tcp.

  • $pw_emu_channel_path{channel-name}: replaces the statement with the path for the given channel name; the channel type should be pty.

Besides running QEMU and renode as the main emulator, the target configuration allows users to start other programs before or after starting the main emulator process. This allows extending the emulated target with simulation or emulation outside of the main emulator. For example, for BLE emulation the main emulator could emulate just the serial port while the HCI emulation done is in an external program (e.g. bumble, netsim).

Command line interface#

The command line interfaces enables users to control emulator instances and access their channels interactively.

usage: pw emu [-h] [-i STRING] [-w WDIR] {command} ...

Pigweed Emulators Frontend

 start               Launch the emulator and start executing, unless pause
                     is set.
 restart             Restart the emulator and start executing, unless pause
                     is set.
 run                 Start the emulator and connect the terminal to a
                     channel. Stop the emulator when exiting the terminal
 stop                Stop the emulator
 load                Load an executable image via gdb. If pause is not set
                     start executing it.
 reset               Perform a software reset.
 gdb                 Start a gdb interactive session
 prop-ls             List emulator object properties.
 prop-get            Show the emulator's object properties.
 prop-set            Show emulator's object properties.
 gdb-cmds            Run gdb commands in batch mode.
 term                Connect with an interactive terminal to an emulator

optional arguments:
 -h, --help            show this help message and exit
 -i STRING, --instance STRING
                       instance to use (default: default)
 -w WDIR, --wdir WDIR  path to working directory (default: None)

commands usage:
    usage: pw emu start [-h] [--file FILE] [--runner {None,qemu,renode}]
                  [--args ARGS] [--pause] [--debug] [--foreground]
     usage: pw emu restart [-h] [--file FILE] [--runner {None,qemu,renode}]
                   [--args ARGS] [--pause] [--debug] [--foreground]
     usage: pw emu stop [-h]
     usage: pw emu run [-h] [--args ARGS] [--channel CHANNEL]
                   {qemu-lm3s6965evb,qemu-stm32vldiscovery,qemu-netduinoplus2} FILE
     usage: pw emu load [-h] [--pause] FILE
     usage: pw emu reset [-h]
     usage: pw emu gdb [-h] [--executable FILE]
     usage: pw emu prop-ls [-h] path
     usage: pw emu prop-get [-h] path property
     usage: pw emu prop-set [-h] path property value
     usage: pw emu gdb-cmds [-h] [--pause] [--executable FILE] gdb-command [gdb-command ...]
     usage: pw emu term [-h] channel

For example, the run command is useful for quickly running ELF binaries on an emulated target and seeing / interacting with a serial channel. It starts an emulator, loads an images, connects to a channel and starts executing.

$ pw emu run qemu-netduinoplus2 out/stm32f429i_disc1_debug/obj/pw_snapshot/test/cpp_compile_test.elf

--- Miniterm on serial0 ---
--- Quit: Ctrl+] | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
INF  [==========] Running all tests.
INF  [ RUN      ] Status.CompileTest
INF  [       OK ] Status.CompileTest
INF  [==========] Done running all tests.
INF  [  PASSED  ] 1 test(s).
--- exit ---

Multiple emulator instances can be run and each emulator instance is identified by its working directory. The default working directory for pw emu is $PW_PROJECT_ROOT/.pw_emu/<instance-id> where <instance-id> is a command line option that defaults to default.

For more complex usage patterns, the start command can be used which will launch an emulator instance in the background. Then, the user can debug the image with the gdb command, connect to a channel (e.g. serial port) with the term command, reset the emulator with the reset command, inspect or change emulator properties with the prop-ls, prop-get, prop-set and finally stop the emulator instance with stop.

Python APIs#

The pw_emu module offers Python APIs to launch, control and interact with an emulator instance.

The following is an example of using these APIs which implements a simplified version of the run pw_emu CLI command:

# start an emulator instance and load the image to execute
# pause the emulator after loading the image
emu = Emulator(args.wdir)
emu.start(args.target, args.file, pause=True)

# determine the channel type and create a pyserial compatible URL
chan_type = emu.get_channel_type(args.chan)
if chan_type == 'tcp':
    host, port = emu.get_channel_addr(chan)
    url = f'socket://{host}:{port}'
 elif chan_type == 'pty':
     url =  emu.get_channel_path(chan)
     raise Error(f'unknown channel type `{chan_type}`')

# open the serial port and create a miniterm instance
serial = serial_for_url(url)
serial.timeout = 1
miniterm = Miniterm(serial)
miniterm.raw = True

# now that we are connected to the channel we can unpause
# this approach will prevent and data loses

except KeyBoardInterrupt:

For convenience, a TemporaryEmulator class is also provided.

It manages emulator instances that run in temporary working directories. The emulator instance is stopped and the working directory is cleared when the with block completes.

It also supports interoperability with the pw emu cli, i.e. starting the emulator with the CLI and controlling / interacting with it from the API.

Usage example:

# programmatically start and load an executable then access it
with TemporaryEmulator() as emu:
    emu.start(target, file)
    with emu.get_channel_stream(chan) as stream:

 # or start it form the command line then access it programmatically
 with TemporaryEmulator() as emu:
         "--run_under=pw emu start <target> --file "

     with emu.get_channel_stream(chan) as stream:

Intended API shape#

This is not an API reference, just an example of the probable shape of the final API.

class Emulator is used to launch, control and interact with an emulator instance:

def start(
    target: str,
    file: os.PathLike | None = None,
    pause: bool = False,
    debug: bool = False,
    foreground: bool = False,
    args: str | None = None,
) -> None:

Start the emulator for the given target.

If file is set that the emulator will load the file before starting.

If pause is True the emulator is paused until the debugger is connected.

If debug is True the emulator is run in foreground with debug output enabled. This is useful for seeing errors, traces, etc.

If foreground is True the emulator is run in foreground otherwise it is started in daemon mode. This is useful when there is another process controlling the emulator’s life cycle (e.g. cuttlefish)

args are passed directly to the emulator

def running(self) -> bool:

Check if the main emulator process is already running.

def stop(self) -> None

Stop the emulator

def get_gdb_remote(self) -> str:

Return a string that can be passed to the target remote gdb command.

def get_gdb(self) -> str | None:

Returns the gdb command for current target.

def run_gdb_cmds(
    commands : list[str],
    executable: Path | None = None,
    pause: bool = False
) -> subprocess.CompletedProcess:

Connect to the target and run the given commands silently in batch mode.

The executable is optional but it may be required by some gdb commands.

If pause is set do not continue execution after running the given commands.

def reset() -> None

Performs a software reset

def list_properties(self, path: str) -> List[dict]

Returns the property list for an emulator object.

The object is identified by a full path. The path is target specific and the format of the path is emulator specific.

QEMU path example: /machine/unattached/device[10]

renode path example: sysbus.uart

def set_property(path: str, prop: str, value: str) -> None

Sets the value of an emulator’s object property.

def get_property(self, path: str, prop: str) -> None

Returns the value of an emulator’s object property.

def get_channel_type(self, name: str) -> str

Returns the channel type.

Currently pty or tcp are the only supported types.

def get_channel_path(self, name: str) -> str:

Returns the channel path. Raises InvalidChannelType if this is not a PTY channel.

def get_channel_addr(self, name: str) -> tuple:

Returns a pair of (host, port) for the channel. Raises InvalidChannelType if this is not a tcp channel.

def get_channel_stream(
    name: str,
    timeout: float | None = None
) -> io.RawIOBase:

Returns a file object for a given host exposed device.

If timeout is None than reads and writes are blocking. If timeout is zero the stream is operating in non-blocking mode. Otherwise read and write will timeout after the given value.

def get_channels(self) -> List[str]:

Returns the list of available channels.

def cont(self) -> None:

Resume the emulator’s execution

Problem investigation#

Pigweed is missing a tool for basic emulators control and as shown in the motivation section directly using emulators directly is difficult.

While emulation is not a goal for every project, it is appealing for some due to the low cost and scalability. Offering a customizable emulators frontend in Pigweed will make this even better for downstream projects as the investment to get started with emulation will be lower - significantly lower for projects looking for basic usage.

There are two main use-cases that this proposal is addressing:

  • Easier and robust interactive debugging and testing on emulators.

  • Basic APIs for controlling and accessing emulators to help with emulator based testing (and trivial CI deployment - as long as the Pigweed bootstrap process can run in CI).

The proposal focuses on a set of fairly small number of commands and APIs in order to minimize complexity and gather feedback from users before adding more features.

Since the state of emulated boards may different between emulators, to enable users access to more emulated targets, the goal of the module is to support multiple emulators from the start.

Two emulators were selected for the initial implementation: QEMU and renode. Both run on all Pigweed currently supported hosts (Linux, Mac, Windows) and there is good overlap in terms of APIs to configure, start, control and access host exposed channels to start with the two for the initial implementation. These emulators also have good support for embedded targets (with QEMU more focused on MMU class machines and renode fully focused on microcontrollers) and are widely used in this space for emulation purposes.

Prior art#

While there are several emulators frontends available, their main focus is on graphical interfaces (aqemu, GNOME Boxes, QtEmu, qt-virt-manager, virt-manager) and virtualization ( virsh, guestfish). qemu-init is a qemu CLI frontend but since it is written in bash it does not work on Windows nor is easy to retrofit it to add Python APIs for automation.

The QEMU project has a few Python modules that are used internally for testing and debugging QEMU. qemu.machine.QEMUMachine implements a QEMU frontend that can start a QEMU process and can interact with it. However, it is clearly marked for internal use only, it is not published on pypi or with the QEMU binaries. It is also not as configurable for pw_emu’s use-cases (e.g. does not support running the QEMU process in the background, does not multiple serial ports, does not support configuring how to expose the serial port, etc.). The qemu.qmp module is published on pypi and can be potentially used by pw_emu to interact with the emulator over the QMP channel.

Detailed design#

The implementation supports QEMU and renode as emulators and works on Linux, Mac and Windows.

Multiple instances are supported in order to enable developers who work on multiple downstream Pigweed projects to work unhindered and also to run multiple test instances in parallel on the same machine.

Each instance is identified by a system absolute path that is also used to store state about the running instance such as pid files for running processes, current emulator and target, etc. This directory also contains information about how to access the emulator channels (e.g. socket ports, PTY paths)

graph TD; TemporaryEmulator & pw_emu_cli[pw emu cli] <--> Emulator Emulator <--> Launcher & Connector Launcher <--> Handles Connector <--- Handles Launcher <--> Config Handles --Save--> WD --Load--> Handles WD[Working Directory]

The implementation uses the following classes:

  • pw_emu.Emulator: the user visible APIs

  • pw_emu.core.Launcher: an abstract class that starts an emulator instance for a given configuration and target

  • pw_emu.core.Connector: an abstract class that is the interface between a running emulator and the user visible APIs

  • pw_emu.core.Handles: class that stores specific information about a running emulator instance such as ports to reach emulator channels; it is populated by pw_emu.core.Launcher and saved in the working directory and used by pw_emu.core.Connector to access the emulator channels, process pids, etc.

  • pw_emu.core.Config: loads the pw_emu configuration and provides helper methods to get and validate configuration options

Documentation update#

The following documentation should probably be updated to use pw emu instead of direct QEMU invocation: pw_rust, lm3s6965evb-qemu. The referenced QEMU targets are defined in fragment configuration files in the pw_emu module and included in the top level pigweed.json file.


UNIX sockets were investigated as an alternative to TCP for the host exposed channels. UNIX sockets main advantages over TCP is that it does not require dynamic port allocation which simplifies the bootstrap of the emulator (no need to query the emulator to determine which ports were allocated). Unfortunately, while Windows supports UNIX sockets since Win10, Python still does not support them on win32 platforms. renode also does not support UNIX sockets.

Open questions#

Renode dynamic ports#

While renode allows passing 0 for ports to allocate a dynamic port, it does not have APIs to retrieve the allocated port. Until support for such a feature is added upstream, the following technique can be used to allocate a port dynamically:

sock = socket.socket(socket.SOCK_INET, socket.SOCK_STREAM)
sock.bind(('', 0))
_, port = socket.getsockname()

There is a race condition that allows another program to fetch the same port, but it should work in most light use cases until the issue is properly resolved upstream.

qemu_gcc target#

It should still be possible to call QEMU directly as described in lm3s6965evb-qemu however, once pw_emu is implemented it is probably better to define a lm3s6965evb emulation target and update the documentation to use pw emu instead of the direct QEMU invocation.