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Re: Interested in contributing to QEMU

From: John Snow
Subject: Re: Interested in contributing to QEMU
Date: Mon, 8 Feb 2021 10:13:35 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 2/7/21 12:01 PM, Niteesh G. S. wrote:
Hello all,

I am Niteesh, a junior student(3rd year) pursuing Electronics and Communication
engineering. I was also a GSoC student for RTEMS last year. My main area of
interest is low-level development (OS, Emulators, Hardware design, etc).

QEMU certainly is the right place for low-level!

I wanted to start contributing from last year itself but was occupied with academic work. I have started working on small patches. My ultimate goal is to learn about
how QEMU works, contribute and learn as much as possible.

I tried going through the Arduino emulation code. I was able to understand it from a high level but couldn't understand underlying details. I went through few blog posts related to QEMU internals but they didn't help much. I plan to step through the code but the sheer size of the codebase is scary(Tips regarding debugging are
very much welcomed). AFAIK the source code is mostly the documentation for
QEMU. If someone knows any docs or articles that will help a beginner get started
it would be great.

Depending on what you'd like to debug, the debugging tips are going to be different. I am not sure of the quality of our Arduino code as I have not touched it personally.

Anything concrete you'd like to ask about how QEMU works? You can always stop by the IRC channel to ask some questions if you're afraid of cluttering up the email list.

See the end of this mail for instructions on how to join, if you need them.

I would also like to take part in GSoC this year. I find the below two projects interesting 1) https://wiki.qemu.org/Google_Summer_of_Code_2020#QEMU_emulated_Arduino_board_visualizer <https://wiki.qemu.org/Google_Summer_of_Code_2020#QEMU_emulated_Arduino_board_visualizer>
This one is from last year AFAIK no one has worked on it. If so I would like
to work on it. I have CC'ed the mentors of this project to share some more details regarding it. Have you guys decided on the netlist parser lib, UI lib? Is there something that I could work on or read to get myself familiarized with the JSON event IO stuff?

I don't think we have the events this project described in the latest development branch of QEMU, but maybe Phil has something in development somewhere. I'll let him answer you, but, it's likely they intend to use QMP here, which is relevant below, too:

2) https://wiki.qemu.org/Google_Summer_of_Code_2021#Interactive.2C_asynchronous_QEMU_Machine_Protocol_.28QMP.29_text_user_interface_.28TUI.29 <https://wiki.qemu.org/Google_Summer_of_Code_2021#Interactive.2C_asynchronous_QEMU_Machine_Protocol_.28QMP.29_text_user_interface_.28TUI.29> This is something that I don't know much about. I have a basic idea about what QMP is but I never used it. The docs say that the Async QMP library is a work in progress. If someone can hook me up with some small tasks in this library it would
be really helpful in improving my understanding.

1) About QMP and QAPI:

QMP is the JSON-like runtime protocol that QEMU supports. These two documents should get you started on understanding what QMP is and how it works:



There's a python script (scripts/qmp/qmp-shell) that can be used to issue an unsupported* short-hand syntax that translates into "real" QMP. Or, as qmp-intro.txt says, you can use telnet or socat to copy/paste JSON straight into the socket.

The commands that QMP accepts are defined by QAPI; those are defined in ./qapi/*.json -- a component called the QAPI generator digests this information and generates the runtime server code that handles parsing and dispatch.

2) About QMP Events

Normally, after the initial handshake, QMP is a call-and-response protocol. The client sends an RPC request, the server executes the command and sends the response.

QMP also supports asynchronous events, though. At any point, QMP can send an unprompted status message that informs the client of some state change within QEMU. This is seen most often for changes in the system emulator runstate, i.e. if QEMU is paused or resumed, etc.

(It's also used for reporting block device errors, long-running task status updates like backup, etc.)

It's likely that Phil wants to use this functionality to send information about GPIO state changes such that a client can render a meaningful visualization.

3) About AQMP

QEMU today has a QMP library written in Python at ./python/qemu/qmp.py. This is a synchronous library that blocks execution until the command is done executing on the server.

We have augmented it over the years to support caching events we received while waiting for execution to finish (for later retrieval), but it requires the caller to go back and check those cached events. It does not offer event handling callbacks. It is a very low-level library that is prone to race conditions depending on how the test using it is written.

qmp-shell uses the old qmp library: it does not show you incoming events as they happen. You have to press "enter" with an empty buffer to coax the shell to check for new events and print them for you, which can be a little annoying if you want live updates.

I started writing AQMP using Python's asyncio/await keywords to create a more modern, flexible QMP library to replace it. One of the hopes I have for this library is that it will handle asynchronous events much more nicely. It's my hope that qmp-shell can be upgraded to use this newer library instead.

The AQMP library is about half finished: It supports all of the basic functionality of the protocol, but needs work on the callback API for dispatching event responses. It also likely needs a lot of testing and polish that's likely to become obvious as anyone tries to integrate it into a real program like a theoretical qmp-shell-2.0.

4) Understanding my project

If you're still interested in my project, I'd recommend trying out qmp-shell against a running QEMU instance and issuing a few basic, boring commands ("query-status" is a good candidate) and seeing how that works.

Then, I'd take a look at some of the other projects I mentioned (mitmproxy, irssi) to get a sense of what the work is here. This is largely a UI/API programming task, and there's real work to do on the AQMP library, but it's probably closer to the surface than the deep technical internals of QEMU.

It might be a good introductory project that helps you get a better overview of the internals of QEMU if you're interested in more hardware-related aspects, but it still requires you have at least some interested in UI programming and API design.

Phil's project might involve hardware specifics quite a bit more than mine, while still teaching you some overview of QMP as a necessity of the project.

If you remain interested after the above, I can point you towards some more concrete tasks that need doing for you to get a fairly concrete sense of what the project entails.

I would like to work on these projects even outside of GSoC if someone is ready to
mentor in their free time :).

Feel free to join #qemu-gsoc on irc.oftc.net. If you've not joined an IRC channel before, it's kind of like a prehistoric slack channel.

Linux GUI: xchat, hexchat
Linux TUI: irssi, WeeChat

OSX GUI: LimeChat, Colloquy (I've never used either)
OSX TUI: irssi and weechat should be available via ports (Not tried.)

Windows GUI: mIRC, XChat

I'm jsnow on OFTC. You can use my nickname at the start of a message ("jsnow: Hello, this is Niteesh from the mailing list") and it will show me a notification -- but the hours I am paying attention to IRC are around 10AM - 7PM EST. (15:00 - 00:00 GMT)

I can be around later by request (00:00 - 05:00 GMT) if you give me some advance notice.


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