cranelift/docs/WASI-background.md

One of the biggest challenges in WebAssembly is figuring out what it's supposed to be.

A brief tangent on some related history

The LLVM WebAssembly backend has gone down countless paths that it has ended up abandoning. One of the early questions was whether we should use an existing object file format, such as ELF, or design a new format.

Using an existing format is very appealing. We'd be able to use existing tools, and be familiar to developers. It would even make porting some kinds of applications easier. And existing formats carry with them decades of "lessons learned" from many people in many settings, building, running, and porting real-world applications.

The actual WebAssembly format that gets handed to platforms to run is its own format, but there'd be ways to make things work. To reuse existing linkers, we could have a post-processing tool which translates from the linker's existing output format into a runnable WebAssembly module. We actually made a fair amount of progress toward building this.

But then, using ELF for example, we'd need to create a custom segment type (in the PT_LOPROC-PT_HIPROC range) instead of the standard PT_LOAD for loading code, because WebAssembly functions aren't actually loaded into the program address space. And same for the PT_LOAD for the data too, because especially once WebAssembly supports threads, memory initialization will need to work differently. And we could omit the PT_GNU_STACK, because WebAssembly's stack can't be executable. And maybe we could omit PT_PHDR because unless we replicate the segment headers in data, they won't actually be accessible in memory. And so on.

And while in theory everything can be done within the nominal ELF standard, in practice we'd have to make major changes to existing ELF tools to support this way of using ELF, which would defeat many of the advantages we were hoping to get. And we'd still be stuck with a custom post-processing step. And it'd be harder to optimize the system to take advantage of the unique features of WebAssembly, because everything would have to work within this external set of constraints.

So while the LLVM WebAssembly backend started out trying to use ELF, we eventually decided to back out of that and design a new format.

Now let's talk APIs

It's apparent to anyone who's looked under the covers at Emscripten's interface between WebAssembly and the outside world that the current system is particular to the way Emscripten currently works, and not well suited for broader adoption. This is especially true as interest grows in running WebAssembly outside of browsers and outside of JS VMs.

It's been obvious since WebAssembly was just getting started that it'd eventually want some kind of "system call"-like API, which could be standardized, and implemented in any general-purpose WebAssembly VM.

And while there are many existing systems we could model this after, POSIX stands out, as being a vendor-neutral standard with considerable momentum. Many people, including us, have been assuming that WebAssembly would eventually have some kind of POSIX API. Some people have even started experimenting with what this might look like.

But while a lot of things map fairly well, some things are less clear. One of the big questions is how to deal with the concept of a "process". POSIX's IPC mechanisms are designed around process, and in fact, the term "IPC" itself has "process" baked into it. The way we even think about what "IPC" means bakes in in understandings about what processes are and what communication between them looks like.

Pipes, Unix-domain sockets, POSIX shared memory, signals, files with fcntl F_SETLK/F_GETLK-style locking (which is process-associated), are are tied to processes. But what is a process, when we're talking about WebAssembly?

Stick a fork in it

Suppose we say that a WebAssembly instance is a "process", for the purposes of the POSIX API. This initially seems to work out well, but it leaves us with several holes to fill. Foremost is fork. fork is one of the pillars of Unix, but it's difficult to implement outside of a full Unix-style OS. We probably can make it work in all the places we want to run WebAssembly, but do we want to? It'd add a bunch of complexity, inefficiency, subtle behavioral differences, or realistically, a combination of all three.

Ok, so maybe we can encourage applications to use posix_spawn instead. And some already do, but in doing so we do lose some of the value of POSIX's momentum. And even with posix_spawn, many applications will explicitly do things like waitpid on the resulting PID. We can make this work too, but we should also take a moment and step back to think about IPC in general.

In WebAssembly, instances can synchronously call each other, and it can be very efficient. This is not something that typical processes can do. Arguably, a lot of what we now think of as "IPC" is just working around the inability of processes to have calls between each other. And, WebAssembly instances will be able to import each others' memories and tables, and eventually even pass around slices to their memories. In WebAssembly circles we don't even tend to think of these as IPC mechanisms, because the process metaphor just doesn't fit very well here. We're going to want applications to use these mechanisms, because they're efficient and take advantage of the platform, rather than using traditional Unix-style IPC which will often entail emulation and inefficiencies.

Of course, there will always be a role for aiding porting of existing applications. Libraries that emulate various details of Unix semantics are valuable. But we can consider them tools for solving certain practical problems, rather than the primary interfaces of the system, because they miss out on some of the platform's fundamental features.

Mm-Mm Mmap

Some of the fundamental assumptions of mmap are that there exists a relatively large virtual address space, and that unmapped pages don't occupy actual memory. The former doesn't tend to hold in WebAssembly, where linear address spaces tend to be only as big as necessary.

For the latter, would it be possible to make a WebAssembly engine capable of unmapping pages in the middle of a linear memory region, and releasing the resources? Sure. Is this a programming technique we want WebAssembly programs doing in general, requiring all VMs to implement this? Probably not.

What's emerging is a sense that what we want is a core set of APIs that can be implemented very broadly, and then optional API modules that VMs can opt into supporting if it makes sense for them. And with this mindset, mmap feels like it belongs in one of these optional sets, rather than in the core.

(although note that even for the use case of reading files quickly, mmap isn't always better than just reading into a buffer.

A WebAssembly port of Debian?

This is a thought-experiment. Debian is ported to numerous hardware architectures. WebAssembly in some settings is presented as a hardware architecture. Would it make sense to port the Debian userspace to WebAssembly? What would this look like? What would it be useful for?

It would be kind of cool to have a WebAssembly-powered Unix shell environment or even a graphical desktop environment running inside a browser. But would it be really cool? Significantly more cool than, say, an SSH or VNC session to an instance in the cloud? Because to do much with it, you'll want a filesystem, a network stack, and so on, and there's only so much that browsers will let you do.

To be sure, it certainly would be cool. But there's a tendency in some circles to think of something like Debian as the natural end goal in a system API and toolchain for WebAssembly. We feel this tendency too ourselves. But it's never really been clear how it's supposed to work.

The insight here is that we can split the design space, rather than trying to solve everything at once. We can have a core set of APIs that will be enough for most applications, but that doesn't try to support all of Debian userland. This will make implementations more portable, flexible, testable, and robust than if we tried to make every implementation support everything, or come up with custom subsets.

As mentioned above, there is room for additional optional APIs to be added beyond the core WASI set. And there's absolutely a place for tools and libraries that features that aren't in the standard platform. So people interested in working on a Debian port can still have a path forward, but we don't need to let this become a focus for the core WASI design.

A picture emerges

While much of what's written here seems relatively obvious in retrospect, this clarity is relatively new. We're now seeing many of the ideas which have been swirling around, some as old as WebAssembly itself, come together into a cohesive overall plan, which makes this an exciting time.