zkburke/risczig

A RISC-V interpreter written in zig

A RISC-V interpreter written in zig

Introduction

A pure Zig implementation of an RV64IAMFD-ziscr-zfence interpreter with the intent of being embedded and used for plugin/mod/scripting systems.

Whilst this project has been written from the ground up in Zig, a huge intellectual debt is owed to https://github.com/fwsGonzo/rvscript for inspiring me to try this out. I think the main innovation that Zig can bring is a much better host-side binding system, where C ABI functions have their ABI implemented and validated at compile time, which should result in faster native calls. This is already partially implemented.

The other thing that makes this implementation differ (currently) is the emulated code's ability to write and read directly from the host memory address space. This is done in a pretty lazy and unsecure way at the moment, but any exploits that could arise from this are already prohibitively difficult to do as the loader implementation essentially has ASLR (Address Space Layout Randomization) in its simplest form.

This project can be thought of as an exploratory leap from my other project that attempted to achieve this in a way that I think RISC-V does better https://github.com/zkburke/callisto

Instruction Set and Extensions

Below are a list of extensions that will be supported. All of these extensions are the unprivileged instruction sets, as risczig implements the unprivileged mode, as the guest program is treated as a user mode program, and the host that the VM is embedded in is modelled as the supervisor.

• RV64I - 64 bit Base Integer Instruction Set
• M - Standard Extension for Integer Multiplication and Division
• A - Standard Extension for Atomic Instructions
• F - Standard Extension for Single-Precision Floating-Point
• D - Standard Extension for Double-Precision Floating-Point
• ZiFencei - Instruction-Fetch Fence

Building and Usage

risczig can be used with the Zig package manager. It has no non Zig dependencies. It is meant to be used as a library and embedded into another application.

Testing

Test the Vm ISA using the riscv-tests repo. Requires cloning https://github.com/riscv-software-src/riscv-tests.git into 'test/lib/'.

zig build test -Dinclude_riscv_tests=true