Slices in WIT · wasm · Zulip Chat Archive

In my current WIP wasm component to manage a wayland compositor I've been thinking of integrating wgpu under the hood but I realize some things like Buffer::map will require a copy between the runtime and the host. I was wondering if there was any proposal to allow WIT to describe a host owned slice (or block of memory) that the runtime can write into?

I'd think something like a slice<u8>. In my use case with wgpu I'd probably make the buffer resource have a function like func slice() -> option<slice<u8>> to get the slice.

However I do realize that there might be a use case for the runtime owning a slice vs always having it borrowed out like in Rust.

i509VCB (Oct 09 2023 at 00:09):

Actually it might be a footgun to have a function the runtime calls to get a slice from inside the host? In rust this would probably double borrow the state?

i509VCB (Oct 09 2023 at 00:20):

Wasm does appear to have untyped memory, but I imagine that exposing that raw memory is a huge footgun

Alex Crichton (Oct 09 2023 at 03:06):

As you've seen WIT doesn't currently have a slice type and there currently isn't a design/proposal to add one. If you're interested in this though opening an issue on the component-model repository with more information might be a good next step. It's unlikely this will be added in the near future though

i509VCB (Oct 09 2023 at 03:24):

I guess for now is there an unsafe way of sorts to have the host give the runtime raw memory via the component model?

Alex Crichton (Oct 09 2023 at 04:43):

Wasmtime has a WasmVec type for this but that's the closest you'll get and it's not easy to use

Dan Gohman (Oct 09 2023 at 15:54):

Another option is to model the buffer as a resource, which lets you pass around handles. That avoids the copying, but the downside is that you can't directly load/store to the buffer from the API; all accesses must go through function calls.

i509VCB (Oct 09 2023 at 16:07):

resource buffer {
    get: func(offset: u64) -> u8

    set: func(offset: u64, write: u8)
}

If a buffer looked like this, you'd be limited to single reads and writes which would definitely result in a huge amount of context switches in and out of the wasm runtime

Stream: wasm

Topic: Slices in WIT