Making a RISC-V Operating System Using Rust
web.eecs.utk.edu
web.eecs.utk.edu
Next I'm trying to convince him to make an OS in Zig for Raspberry Pi with a focus on graphics. If you have any suggestions for him, shoot him an email!
https://github.com/ziglang/zig/issues/3211
BUT the sokol_app.h and sokol_gfx.h headers require an underlying platform 3D-API (e.g. GL, D3D11 or Metal) and window system to setup the swap chain. On a bare metal machine with only a 2D framebuffer they're not all that useful unfortunately.
The standard library assumes infallible allocation currently. Kernels generally do not use the standard library.
(Okay, STRICTLY speaking there’s a hack for Box<T> in the language right now but it’s not about the allocation part and Box<T> is technically defined in the standard library with Magic(tm) and so isn’t relevant in an OS dev context, as you won’t use the standard library and therefore Box<T>. Rust-the-language never implicitly inserts heap allocations, including Box<T>, anywhere.)
Saying that Rust the language doesn't require a non-failing allocator misses the point--the ergonomics of the language make dealing with allocation failure difficult; sufficiently difficult that none of the projects I've seen actually bother attempting it.
See https://cs.brown.edu/research/pubs/theses/ugrad/2015/light.a..., which explains idiomatic Rust instructs developers to return by value, relying on caller assignment to types like Box (which uses exchange_malloc under the hood), to handle heap allocation. Basically, the strategy for dynamic object management in Rust is predicated on hidden heap allocations.
So of course it's not necessary. But good luck writing an entire operating system otherwise. Even Redox OS doesn't bother trying to fight the language in this regard: https://gitlab.redox-os.org/redox-os/slab_allocator/blob/mas...
Honest question: what languages make this ergonomic and can you share any projects that handle this gracefully?
What makes it a potential impediment in Rust is that the constraints and burdens of the borrow checker are offset by mechanisms like Box. The fact that all the extant examples choose the convenience of Box over handling OOM, even in situations where not handling OOM is obviously a deal breaker for production systems, speaks volumes about the significance of the problem and how the language shapes people's choices. Async/await is in the same boat--technically doesn't require a non-failing heap allocation, but who's going to bother making it work? You technically don't need async/await to do asynchronous programming, either, but the whole point was that this is the type of thing that needs to be addressed by the core language with some primitive (i.e. generators) that does the heavy lifting and which can be built upon.
I don't know enough Rust to know how easy it would be to create a Box-like implementation that rewrites the AST to automatically propagate allocation failure via the idiomatic Result<T, E> protocol. But that seems roughly what the proper solution might look like in Rust; either that or finally getting over the anxiety and aversion about exceptions.
Lua handles OOM quite well. Lua doesn't have try/catch, just "protected calls" which are not as light-weight as regular function calls. (A pcall initializes a recovery point with _setjmp.) In Lua you tend to use protected calls at, effectively, transactional boundaries--the points in your call graph where you're willing and able to rollback application state for non-specific, otherwise unrecoverable errors. AFAIU you could technically do the same in Rust, except Rust makes unwinding optional at compile-time[1], so it's not the kind of thing people will make a habit of deliberately designing for in their libraries. Which makes me think the likely solution for Rust, if any, is to permit libraries to opt-in to OOM recovery with an allocator pragma that does something like the Result<T, E> propagation mentioned above.
[1] Lua is GC'd. The cost of running destructors outside the normal call/return protocol is fixed and independent of whether an application uses protected calls.
One is the `box` keyword: this guarantees that the object will be constructed in place on the heap, but doesn't work with fallible allocators (or, in fact, anything but the default allocator). This is what the research paper you linked is talking about. However, all these years later, `box` has never been stabilized; nor will it be in its current form, because it's considered too inflexible. Whatever form of in-place construction does eventually get stabilized will likely support fallibility.
The other way to allocate a Box is `Box::new`. This is not compiler magic; it's simply a regular function, implemented in Rust, that calls the allocator and then moves (i.e. memcpys) an existing object into the new allocation. If you write your own Box-like type, there's nothing stopping you from making your `new` function fallible.
What about optimizations? Does `Box::new` get optimized in ways that a fallible version won't? Well, no. The compiler will inline `Box::new`, and if you call it with a fresh stack allocation as an argument, LLVM can theoretically, sometimes, elide the stack allocation and the memcpy altogether, instead initializing the object directly on the heap. Theoretically. The paper claims that it always does so, but the paper is wrong. [1] In fact, the compiler doesn't do so even in relatively easy cases. [2] It would be nice if LLVM did better here, but it doesn't seem to be a big source of overhead in Rust programs in practice. If LLVM did improve the optimization, it would probably work equally well for a fallible allocator as for `Box::new`, because they're equally complex from its perspective: `Box::new` can panic, and LLVM treats panics as branches.
(Box does have compiler magic for a different case: the ability to move out of it. Not being able to replicate that in a custom type is suboptimal, but not the end of the world.)
As for why those OS projects panic on allocation failure:
The phil-opp.com one implements the standard allocator interface in order to use the standard library container types. I think it sucks that Rust's standard containers don't support allocation failure, but you don't need to use them...
For Redox... I'm not actually sure what they're doing, but I think "fn oom" is a hook for users of the allocator to call if they want to panic on out-of-memory, not something that mandates panicking. At least, that's the behavior of `std::alloc::handle_alloc_error` [3], which was moved there from being a trait method on `GlobalAlloc` named `oom`. However, they're implementing a different `oom` on an old version of the `Alloc` (not `GlobalAlloc`) trait, which is unstable; that method was removed entirely well over a year ago, so I guess the code must be out of date.
[1] https://users.rust-lang.org/t/how-to-create-large-objects-di...
[2] https://play.rust-lang.org/?version=stable&mode=release&edit... (press "..." -> "ASM")
[3] https://doc.rust-lang.org/nightly/std/alloc/fn.handle_alloc_...
You can implement your own global allocator, use an existing crate, or (in the case of user applications) use the system's. That is for the stdlib level, not even core...
Linux doesn't use malloc; a Rust kernel need not use Box::new either.
I subsequently branched out to e.g. `do-backups.sh`, where despite the `do-` being superfluous, I quite like the aesthetic.
Slightly more on topic: I really enjoy blog series like these, with plenty of detail on esoteric topics I really have no idea about, or on the face of it much interest in. They're a fun way to increase the breadth of topics I have a superficial knowledge of.
The first 20% of the effort gets you about 80% of the results. So everything seems exciting. But to that that last 20% of the results requires 80% of the total work.
There is a ton of un-fun, un-glamorous work getting a gazillion device drivers written, for example.
Rump kernels are the closest trend.
Because this is generally not secure. The driver OS will have access to hardware that can bypass the memory restrictions set upon it by the microkernel.
There is sometimes special hardware to address this but they are too complex to manage from the kernel.
Most OS's aren't designed to be that secure, though. That's why I wonder why it hasn't been tried more for usability. A security-focused setup certainly has more to be concerned about. Like I advocated with Xen, a good start would be making the host OpenBSD. They should be able to get hardware that would be compatible.
> RISC-V ("risk five") and the Rust programming language both start with an R, so naturally they fit together
And don't say cobol.
It's not like current "CISC" architectures haven't taken a lot of the good aspects of RISC already too...
Crystal?[2]
Church?[3]
Coq?[4]
[2]: https://crystal-lang.org/
[3]: https://en.wikipedia.org/wiki/Church_(programming_language)
Common Lisp would be a decent choice, too, with
CISC:RISC = Common Lisp:SchemeYou could vlog about it.
Worse is better, forever.