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wadi

Web Assembly Device Interface (wadi) is a complementary specification for wasi hosts that want to offer device drivers. Users may wish to extend/reduce the capabilities of the host environment at startup or runtime. This spec helps standadize the means by which wasi can communicate with web assembly device backed files.

How it works

A host environment must first have a means by which device drive wasm modules are integrated into the running host. Consider a hypothetical device driver cowbell.wasm that plays a bell sound and a hypothetical wasmer environment. We would first start by loading this driver.

libw::write_text("/_wasmer/kernel/driver","/home/richard/cowbell.wasm");

The device driver web assembly exposes a number of external facing functions that will be called by the host environment. Initially it will call init() on the device module.

The responsability of the device module will be to first register the scope of files that will be handled. Cow bell might only want a single file in your wasi host environment ('/dev/cowbell') but other devices, might want to handle whole heirarhies of directories (/dev/usb/*)

Once registered, the device will be usable like any other wasi file.

libw::write_text("/dev/cowbell","play");

wadi makes no restrictions on what additional host functions are exposed to web assembly module, allowing the device to do anything within the host's power it would like to allow.

Example

use wadi::{register_scope,CString,cstr}
use js_ffi::*;

#[no_mangle]
pub fn init() {
    register_scope("/dev/cowbell");
}

#[no_mangle]
pub fn write(path: CString, data_ptr: usize, data_len: usize) {
    js!(window.alert).invoke_1("clonk!");
}

#[no_mangle]
pub fn name() -> usize {
    cstr("cowbell")
}

wadi device interface

• init() - start the device and register scopes
• read(path,location, size, target) - read a certain number of bytes from a file path
• write(path,data,location,size) - write a certain number of bytes from a file
• query(path) - get information on one or many files controlled by the device
• malloc(size) - ask for some memory to be able to write data into
• free(ptr) - free data
• name() - returns a name of the module

wadi host interface

• register_device(file_scope, file_type)
• error(err)