Wasmjit
Wasmjit is a small embeddable WebAssembly runtime. Its core is written in C90 and is easily portable to most environments.
Its primary target is a Linux kernel module that can host Emscripten-generated WebAssembly modules. In this configuration it runs WebAssembly modules in kernel-space (ring 0) and provides access to system calls as normal function calls. This configuration avoids user-kernel transition overhead, as well as scheduling overheads from swapping page tables. This results in a significant performance increase for syscall-bound programs like web servers or FUSE file systems. The performance increase is more pronounced in a post-Meltdown world due to PTI overhead. Check it out:
It also includes a host environment for running in user-space on POSIX systems. This is useful for running WebAssembly modules without having to run an entire browser.
Wasmjit is licensed under a permissive MIT license.
Getting Started
Wasmjit currently runs on x86_64 POSIX systems, like Linux, macOS, and the BSDs. It can also optionally use a Linux kernel module that can significantly speed up execution. To get started with Wasmjit you need a few tools:
- A standard POSIX C development environment that includes
cc
andmake
. - Emscripten SDK
- (Optional) Your Linux distro's kernel headers. On Debian, install
the
linux-headers-amd64
package, on Fedora, installkernel-devel
.
(Optional) Installing the Kernel Module
Wasmjit has been designed for and tested with kernel versions 4.17+, though it should work with reasonably modern kernels. Your mileage may vary.
$ make modclean
$ make kwasmjit.ko
$ sudo insmod ./kwasmjit.ko
$ sudo chmod 777 /dev/wasm # See caveat below
Building the Executable
$ make clean
$ make -j wasmjit
Building and running the Example
$ source path_to_emsdk/emsdk_env.sh # bring emcc into $PATH
$ emcc -o selfpipe.js src/wasmjit_examples/selfpipe.c
You can now run the example:
$ time ./wasmjit selfpipe.wasm
If you installed the Linux kernel module, this should run much quicker than a native binary:
$ cc -o selfpipe src/wasmjit_examples/selfpipe.c
$ time ./selfpipe
Status
Wasmjit can run a subset of Emscripten-generated WebAssembly on Linux, macOS, and within the Linux kernel as a kernel module. It currently only supports x86_64. Here are the current developments goals in order of priority:
- Implement enough Emscripten host-bindings to run
nginx.wasm
- Implement an interpreter
- Implement Rust-runtime for Rust-generated wasm files
- Implement Go-runtime for Go-generated wasm files
- Optimize x86_64 JIT
- Implement arm64 JIT
- Implement JIT debugging symbols
- Implement macOS kernel module
PRs are welcome :) see CONTRIBUTING.md first.
Linux Kernel Mode Caveats
The code and data section allocations are done using vmalloc()
. This
prevents those pages from ever being swapped to disk. Thus,
indiscriminately allowing access to the /dev/wasm
device may make a
system vulnerable to denial-of-service attacks. In the future a
system-wide limit on the amount of memory used by the /dev/wasm
device will be provided to mitigate that risk.
Contact
Rian Hunter @cejetvole