libriscv is a RISC-V emulator that is highly embeddable and configurable. This project is intended to be included in a CMake build system, and should not be installed anywhere. There are several CMake options that control RISC-V extensions and how the emulator behaves.

The emulator has a binary translation mode that has currently only been tested on Linux, but the performance should be good enough with just enabling experimental + icache.

While this emulator has a focus on performance, one higher priority is the ability to map any memory anywhere with permissions, custom fault handlers and such things. This allows you to take the memory of one machine and map it into another, and does have a slight performance penalty compared to an emulator that can only have sequential memory.

One motivation when writing this emulator was to use it in a game engine, and so it felt natural to compare against Lua, which I was already using. Lua is excellent and easy to embed, and does not require ahead-of-time compilation.

STREAM memory benchmark

Lua 5.4 benchmarks

Optimized compiled code is faster even when interpreted.

On Ubuntu and Linux distributions like it, you can install a 64-bit RISC-V GCC compiler for running Linux programs with a one-liner:

sudo apt install gcc-10-riscv64-linux-gnu g++-10-riscv64-linux-gnu

Now you have a full C/C++ compiler for RISC-V. It is typically configured to use the C-extension, so make sure you have that enabled.

To build smaller and leaner programs you will need a (limited) Linux userspace environment. You sometimes need to build this cross-compiler yourself:

git clone https://github.com/riscv/riscv-gnu-toolchain.git
cd riscv-gnu-toolchain
./configure --prefix=$HOME/riscv --with-arch=rv32g --with-abi=ilp32d
make

This will build a newlib cross-compiler with C++ exception support. The ABI is ilp32d, which is for 32-bit and 64-bit floating-point instruction set support. It is much faster than software implementations of binary IEEE floating-point arithmetic.

Note that if you want a full glibc cross-compiler instead, simply appending linux to the make command will suffice, like so: make linux. Glibc is harder to support, and produces larger binaries, but will be more performant. It also supports threads, which is awesome to play around with.

git clone https://github.com/riscv/riscv-gnu-toolchain.git
cd riscv-gnu-toolchain
./configure --prefix=$HOME/riscv --with-arch=rv64g --with-abi=lp64d
make

The incantation for 64-bit RISC-V.

The last step is to add your compiler to PATH so that it becomes visible to build systems. So, add this at the bottom of your .bashrc file in the home (~) directory:

export PATH=$PATH:$HOME/riscv/bin

From one of the binary subfolders:

$ ./build.sh

Which will produce a hello_world binary in the sub-projects build folder.

Building the emulator and booting the newlib hello_world:

cd emulator
mkdir -p build && cd build
cmake .. && make -j4
./rvlinux ../../binaries/linux64/build/hello_world

The emulator is built 3 times for different purposes. rvmicro is built for micro-environments with custom heap and threads. rvnewlib has hooked up enough system calls to run newlib. rvlinux has all the system calls necessary to run a normal userspace linux binary.

Building and running your own ELF files that can run in freestanding RV32GC is quite challenging, so consult the barebones example! It's a bit like booting on bare metal, except you can more easily implement system functions. The fun part is of course the extremely small binaries and total control over the environment.

The newlib example project have much more C and C++ support, but still misses things like environment variables and such. This is a deliberate design as newlib is intended for embedded development. It supports C++ RTTI and exceptions, and is the best middle-ground for running a fuller C++ environment that still produces small binaries.

The full example project uses the Linux-configured cross compiler and will expect you to implement quite a few system calls just to get into int main(). In addition, you will have to setup argv, env and the aux-vector. There is a helper method to do this in the src folder. There is also basic pthreads support.

And finally, the micro project implements the absolutely minimal freestanding RV32GC C/C++ environment. You won't have a heap implementation, so no new/delete. And you can't printf values because you don't have a C standard library, so you can only write strings and buffers using the write system call. Still, the stripped binary is only 784 bytes, and will execute only ~120 instructions running the whole program! The micro project actually initializes zero-initialized memory, calls global constructors and passes program arguments to main.

If you have built the emulator, you can use DEBUG=1 ./emulator /path/to/program to enable GDB to connect. Start your GDB like so: riscv64-unknown-elf-gdb /path/to/program. Make sure your program is built with -O0 and with debuginfo present. Then, once in GDB connect with target remote localhost:2159. Now you can step through the code.

Most modern languages embed their own pretty printers for debuginfo which enables you to go line by line in your favorite language.

The emulator currently supports RV32GC, RV64GC (IMAFDC) and RV128G. The F and D-extensions should be 100% supported (32- and 64-bit floating point instructions), and there is a test-suite for these instructions, however they haven't been extensively tested as there are generally few FP-instructions in normal programs.

The 128-bit ISA support is experimental, and the specification is not yet complete. There is neither toolchain support, nor is there an ELF format for 128-bit machines. There is an emulator that specifically runs a custom crafted 128-bit program in the emulator/emu128 folder.

Binary translation currently only supports RV32G and RV64G.

Note: There is no support for the B-, E-, V- and Q-extensions.

Load a binary and let the machine simulate from _start (ELF entry-point):

#include <libriscv/machine.hpp>

int main(int /*argc*/, const char** /*argv*/)
{
	// Load ELF binary from file
	const std::vector<uint8_t> binary /* = ... */;

	using namespace riscv;

	// Install the `exit` system call handler
	Machine<RISCV64>::install_syscall_handler(93,
	 [] (Machine<RISCV64>& machine) {
		 const int code = machine.return_value <int> ();
		 printf(">>> Program exited, exit code = %d\n", code);
		 machine.stop();
	 });

	// Create a 64-bit machine
	Machine<RISCV64> machine { binary };

	// Add program arguments on the stack
	machine.setup_argv({"emulator", "test!"});

	// This function will run until the exit syscall has stopped the
	// machine, an exception happens which stops execution, or the
	// instruction counter reaches the given limit (1M):
	try {
		machine.simulate(1'000'000);
	} catch (const std::exception& e) {
		fprintf(stderr, ">>> Runtime exception: %s\n", e.what());
	}
}

You can find the example above in the emulator/minimal folder. It's a normal CMake project, so you can build it like so:

mkdir -p build && cd build
cmake .. && make -j4
./emulator

If you run the program as-is with no program loaded, you will get an Execution space protection fault, which means the emulator tried to execute on non-executable memory.

$ ./emulator
>>> Runtime exception: Execution space protection fault

You can limit the amount of (virtual) memory the machine can use like so:

	const uint32_t memsize = 1024 * 1024 * 64;
	riscv::Machine<riscv::RISCV32> machine { binary, { .memory_max = memsize } };

You can limit the amount of instructions to simulate at a time like so:

	const uint64_t max_instructions = 1000;
	machine.simulate(max_instructions);

Similarly, when making a function call into the VM you can also add this limit as a template parameter to the vmcall() function.

You can find details on the Linux system call ABI online as well as in the syscalls.hpp, and syscalls.cpp files in the src folder. You can use these examples to handle system calls in your RISC-V programs. The system calls is emulate normal Linux system calls, and is compatible with a normal Linux RISC-V compiler.

You can create your own custom instruction loop if you want to do things manually by yourself:

#include <libriscv/machine.hpp>
#include <libriscv/rv32i_instr.hpp>
...
auto& cpu = machine.cpu;
while (!machine.stopped()) {
	// Get 32- or 16-bits instruction
	auto instr = cpu.read_next_instruction();
	// Decode instruction to get instruction info
	auto handlers = cpu.decode(instr);
	if (false) {
		// Print instruction to terminal
		auto assembly = cpu.to_string(instr, handlers);
		printf("%.*s\n", (int)assembly.size(), assembly.c_str());
	}
	// Execute one instruction, and increment PC
	handlers.handler(cpu, instr);
	cpu.increment_pc(instr.length());
}

NOTE: Make sure to disable instruction caching when doing this. Some features like instruction fusing will modify instruction bits for performance reasons, which may be only compatible with the instruction cache mechanism, as well as binary translation.

You can create a 64kb machine without a binary, and no ELF loader will be invoked. Make sure you disable experimental features, as they may require you to set up execute segments before you can execute code.

	std::string_view empty;
	riscv::Machine<riscv::RISCV32> machine { empty, { .memory_max = 65536 }};

Now you can copy your machine code directly into memory:

	std::vector<uint8_t> my_program;
	const uint32_t dst = 0x1000;
	machine.copy_to_guest(dst, my_program.data(), my_program.size());
	// We will be making it execute-only (although you may want to enable read)
	machine.memory.set_page_attr(dst, my_program.size(),
		{.read = false, .write = false, .exec = true});

Finally, let's jump to the program entry, and start execution:

	// Example PC start address
	machine.cpu.jump(0x1068);

	// Geronimo!
	machine.simulate(5'000);

The fuzzing program does this, so have a look at that.

System calls

Freestanding environments

Function calls into the VM

Debugging in the VM

It's a drop-in sandbox. Perhaps you want someone to be able to execute C/C++ code on a website, safely?

See the webapi folder for an example web-server that compiles and runs limited C/C++ code in a relatively safe manner. Ping me or create a PR if you notice something is exploitable.

Note that the web API demo uses a docker container to build RISC-V binaries, for security reasons. You can build the container with docker build -t newlib-rv32gc . -f newlib.Dockerfile from the docker folder. Alternatively, you could build a more full-fledged Linux environment using docker build -t linux-rv32gc . -f linux.Dockerfile. There is a test-script to see that it works called dbuild.sh which takes an input code file and output binary as parameters.

It can also be used as a script backend for a game engine, as it's quite a bit faster than LuaJIT, although it requires you to compile the scripts ahead of time as binaries using any computer language which can output RISC-V.

Use Clang (newer is better) to compile the emulator with. It is somewhere between 20-25% faster on most everything.

Use GCC to build the RISC-V binaries. Use -O2 or -O3 and use the regular standard extensions: -march=rv32gc -mabi=ilp32d. Enable the RISCV_EXPERIMENTAL option for the best performance unless you are using libriscv as a sandbox. Use -march=rv32g for the absolute best performance, if you have that choice. Difference is minimal so don't go out of your way to build everything yourself. Always enable the instruction decoder cache as it makes decoding much faster at the cost of extra memory. Always enable LTO if you can.

Building the fastest possible RISC-V binaries for libriscv is a hard problem, but I am working on that in my rvscript repository. It's a complex topic that cannot be explained in one paragraph.

If you have arenas available you can replace the default page fault handler with your that allocates faster than regular heap. If you intend to use many (read hundreds, thousands) of machines in parallel, you absolutely must use the forking constructor option. It will apply copy-on-write to all pages on the newly created machine and share text and rodata. Also, enable RISCV_EXPERIMENTAL so that the decoder cache will be generated ahead of time.

Instead of JIT, the emulator supports translating binaries to native code using any local C or C++ compiler. You can control compilation by passing CC and CFLAGS environment variables to the program that runs the emulator. You can show the compiler arguments using VERBOSE=1. Example: CFLAGS=-O2 VERBOSE=1 ./myemulator.

The binary translation feature (accessible by enabling RISCV_EXPERIMENTAL) can greatly improve performance in some cases, but requires compiling the program on the first run. The RISC-V binary is scanned for code blocks that are safe to translate, and then a C compiler is invoked on the generated code. This step takes a long time. The resulting code is then dynamically loaded and ready to use. The feature is a work in progress.