🧮 uvm32

uvm32 is a minimalist, dependency-free virtual machine sandbox designed for microcontrollers and other resource-constrained devices. Single C file, no dynamic memory allocations, asynchronous design, pure C99.

Features

• Bytecode example apps written in C, Zig, Rust and assembly
• Non-blocking design, preventing misbehaving bytecode from stalling the host
• No assumptions about host IO capabilities (no stdio)
• Simple, opinionated execution model
• Safe minimally typed FFI
• Small enough for "if this then that" scripts/plugins, capable enough for much more

Although based on a fully fledged CPU emulator, uvm32 is intended for executing custom script like logic, not for simulating hardware.

Samples

• host vm host which loads a binary and runs to completion, handling multiple syscall types
• host-mini minimal vm host (shown above), with baked in bytecode
• host-parallel parallel vm host running multiple vm instances concurrently, with baked in bytecode
• host-arduino vm host as Arduino sketch (tested on Arduino Uno ATmega328P, uses 9950 bytes of flash/1254 bytes RAM)
• apps/helloworld C hello world program
• apps/conio C console IO demo
• apps/lissajous C console lissajous curve (showing softfp, floating point)
• apps/hello-asm Minimal hello world assembly
• apps/fib C fibonacci series program (iterative and recursive)
• apps/self host-mini with embedded mandelbrot generation program, compiled as an app (inception!)
• apps/sketch C Arduino/Wiring/Processing type program in setup() and loop() style
• apps/rust-hello Rust hello world program (note, the version of rust installed by brew on mac has issues, use the official rust installer from https://rust-lang.org/learn/get-started/)
• apps/zig-mandel Zig ASCII mandelbrot generator program
• apps/zigtris Zig Tetris (https://github.com/ringtailsoftware/zigtris)

Quickstart

make
host/host precompiled/mandel.bin
host/host precompiled/zigtris.bin

Build sample apps (sets up docker for cross compiler)

cd apps
make

Build one of the sample apps (requires docker for C, or Zig, or Rust)

cd apps/helloworld && make

Run the app

./host ../apps/helloworld/helloworld.bin

Quickstart API

uint8_t bytecode[] = { /* ... */ }; // some compiled bytecode
uvm32_state_t vmst; // execution state of the vm
uvm32_evt_t evt; // events passed from vm to host

uvm32_init(&vmst); // setup vm
uvm32_load(&vmst, bytecode, sizeof(bytecode)); // load the bytecode
uvm32_run(&vmst, &evt, 100); // run up to 100 instructions

switch(evt.typ) {
	// check why the vm stopped executing
}

Operation

Once loaded with bytecode, uvm32's state is advanced by calling uvm32_run().

uint32_t uvm32_run(uvm32_state_t *vmst, uvm32_evt_t *evt, uint32_t instr_meter)

uvm32_run() will execute until the bytecode requests some IO activity from the host. These IO activities are called "syscalls" and are the only way for bytecode to communicate with the host. If the bytecode attempts to execute more instructions than the the passed value of instr_meter it is assumed to have crashed and an error is reported.

(As with a watchdog on an embedded system, the yield() bytecode function tells the host that the code requires more time to complete and has not hung)

uvm32_run() always returns an event. There are four possible events:

• UVM32_EVT_END the program has ended
• UVM32_EVT_ERR the program has encountered an error
• UVM32_EVT_YIELD the program has called yield() signifying that it requires more instructions to be executed, but has not crashed/hung
• UVM32_EVT_SYSCALL the program requests some IO via the host

Internals

uvm32 emulates a RISC-V 32bit CPU using mini-rv32ima. All IO from vm bytecode to the host is performed using ecall syscalls. Each syscall provided by the host requires a unique syscall value. A syscall passes two values and receives one on return.

uvm32 is always in one of 4 states, paused, running, ended or error.

stateDiagram
    [*] --> UVM32_STATUS_PAUSED : uvm32_init()
    UVM32_STATUS_PAUSED-->UVM32_STATUS_RUNNING : uvm32_run()
    UVM32_STATUS_RUNNING --> UVM32_STATUS_PAUSED : syscall event
    UVM32_STATUS_RUNNING --> UVM32_STATUS_ENDED : halt()
    UVM32_STATUS_RUNNING --> UVM32_STATUS_ERROR

Boot

At boot, the whole memory is zeroed. The user program is placed at the start. The stack pointer is set to the end of memory and grows downwards. No heap region is setup and all code is in RAM.

syscall ABI

All communication between bytecode and the vm host is performed via syscalls.

To make a syscall, register a7 is set with the syscall number (a UVM32_SYSCALL_x) and a0, a1 are set with the syscall parameters. The response is returned in a2.

target.h

static uint32_t syscall(uint32_t id, uint32_t param1, uint32_t param2) {
    register uint32_t a0 asm("a0") = (uint32_t)(param1);
    register uint32_t a1 asm("a1") = (uint32_t)(param2);
    register uint32_t a2 asm("a2");
    register uint32_t a7 asm("a7") = (uint32_t)(id);

    asm volatile (
        "ecall"
        : "=r"(a2) // output
        : "r"(a7), "r"(a0), "r"(a1) // input
        : "memory"
    );
    return a2;
}

The RISC-V SBI is not followed, a simpler approach is taken.

syscalls

There are two system syscalls used by uvm32, halt() and yield().

halt() tells the host that the program has ended normally. yield() tells the host that the program requires more instructions to be executed. These are handled internally to uvm32.

Syscalls are handled in the host by reading the syscall identifier, then using the provided functions to get arguments and set a return response. Direct access to the VM's memory space is not allowed, to avoid memory corruption issues.

The following functions are used to access syscall parameters safely:

uint32_t uvm32_getval(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t);
const char *uvm32_getcstr(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t);
void uvm32_setval(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t, uint32_t val);
uvm32_evt_syscall_buf_t uvm32_getbuf(uvm32_state_t *vmst, uvm32_evt_t *evt, uvm32_arg_t argPtr, uvm32_arg_t argLen);

Here is a full example of a working VM host from apps/host-mini

--

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "uvm32.h"
#include "../common/uvm32_common_custom.h"

uint8_t rom[] = { // mandel.bin
  0x23, 0x26, 0x11, 0x00, 0xef, 0x00, 0xc0, 0x00, 0xb7, 0x08, 0x00, 0x01,
  0x73, 0x00, 0x00, 0x00, 0x13, 0x01, 0x01, 0xff, 0x23, 0x26, 0x81, 0x00,
  0x37, 0xf5, 0xff, 0xff, 0xb7, 0x15, 0x00, 0x00, 0x37, 0xe6, 0xff, 0xff,
  0x13, 0x07, 0xf0, 0x01, 0xb7, 0x47, 0x00, 0x00, 0xb7, 0x06, 0x00, 0x01,
  0x13, 0x08, 0xd5, 0xcc, 0x93, 0x82, 0x35, 0x33, 0x13, 0x03, 0x76, 0xe6,
  0x93, 0x83, 0x35, 0xb3, 0x13, 0x86, 0x16, 0x00, 0x63, 0xce, 0x02, 0x09,
  0x13, 0x0e, 0x03, 0x00, 0x63, 0xce, 0x63, 0x06, 0x93, 0x0f, 0x00, 0x00,
  0x13, 0x0f, 0x00, 0x00, 0x13, 0x04, 0x00, 0x00, 0x93, 0x0e, 0x00, 0x00,
  0x93, 0x06, 0x00, 0x02, 0x13, 0x85, 0x06, 0xfe, 0x63, 0x64, 0xa7, 0x04,
  0x33, 0x05, 0xff, 0x01, 0x63, 0xe0, 0xa7, 0x04, 0x13, 0x05, 0x00, 0x00,
  0x93, 0x05, 0x00, 0x00, 0x93, 0x08, 0x06, 0x00, 0xb3, 0x8e, 0x8e, 0x02,
  0x33, 0x0f, 0xff, 0x41, 0x13, 0xd4, 0xbe, 0x40, 0xb3, 0x0e, 0xcf, 0x01,
  0x73, 0x00, 0x00, 0x00, 0x33, 0x04, 0x04, 0x01, 0x33, 0x85, 0xde, 0x03,
  0x13, 0x5f, 0xc5, 0x00, 0x33, 0x05, 0x84, 0x02, 0x93, 0x5f, 0xc5, 0x00,
  0x93, 0x86, 0x16, 0x00, 0x6f, 0xf0, 0x9f, 0xfb, 0x13, 0x85, 0x06, 0x00,
  0x93, 0x05, 0x00, 0x00, 0x93, 0x08, 0x00, 0x00, 0x73, 0x00, 0x00, 0x00,
  0x13, 0x0e, 0x1e, 0x09, 0xe3, 0xd6, 0xc3, 0xf9, 0x13, 0x05, 0xa0, 0x00,
  0x93, 0x05, 0x00, 0x00, 0x93, 0x08, 0x00, 0x00, 0x73, 0x00, 0x00, 0x00,
  0x13, 0x08, 0x98, 0x19, 0xe3, 0xd6, 0x02, 0xf7, 0x37, 0x05, 0x00, 0x80,
  0x13, 0x05, 0x05, 0x10, 0x93, 0x08, 0x30, 0x00, 0x93, 0x05, 0x00, 0x00,
  0x73, 0x00, 0x00, 0x00, 0x03, 0x24, 0xc1, 0x00, 0x13, 0x01, 0x01, 0x01,
  0x67, 0x80, 0x00, 0x00, 0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x77, 0x6f,
  0x72, 0x6c, 0x64, 0x00
};

int main(int argc, char *argv[]) {
    uvm32_state_t vmst;
    uvm32_evt_t evt;
    bool isrunning = true;

    uvm32_init(&vmst);
    uvm32_load(&vmst, rom, sizeof(rom));

    while(isrunning) {
        uvm32_run(&vmst, &evt, 100);   // num instructions before vm considered hung

        switch(evt.typ) {
            case UVM32_EVT_END:
                isrunning = false;
            break;
            case UVM32_EVT_SYSCALL:    // vm has paused to handle UVM32_SYSCALL
                switch(evt.data.syscall.code) {
                    case UVM32_SYSCALL_PUTC:
                        printf("%c", uvm32_getval(&vmst, &evt, ARG0));
                    break;
                    case UVM32_SYSCALL_PRINTLN: {
                        const char *str = uvm32_getcstr(&vmst, &evt, ARG0);
                        printf("%s\n", str);
                    } break;
                    default:
                        printf("Unhandled syscall 0x%08x\n", evt.data.syscall.code);
                    break;
                }
            break;
            case UVM32_EVT_ERR:
                printf("UVM32_EVT_ERR '%s' (%d)\n", evt.data.err.errstr, (int)evt.data.err.errcode);
            break;
            default:
            break;
        }
    }

    return 0;
}

Configuration

The uvm32 memory size is set at compile time with -DUVM32_MEMORY_SIZE=X (in bytes). A memory of 512 bytes will be sufficient for trivial programs.

Debugging

The Makefile in apps builds a docker container with a RISC-V toolchain. This can be used to disassemble with:

docker run -v `pwd`:/data -w /data --rm riscv-dev riscv64-unknown-elf-objdump -d -f hello-asm.elf
docker run -v `pwd`:/data -w /data --rm riscv-dev riscv64-unknown-elf-objdump -S -d -b binary -m riscv:rv32 -D -M no-aliases -f hello-asm.bin

License

This project is licensed under the MIT License. Feel free to use in research, products and embedded devices.