rvsim 1.2.1

High-performance RISC-V cycle-accurate system simulator

rvsim

Cycle-accurate RISC-V 64-bit system simulator with a composable Python API for architecture research and design-space exploration.

Documentation · PyPI · Rust Core (crates.io) · Changelog

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rvsim models a complete superscalar processor at cycle granularity. It implements two pluggable microarchitectural backends — out-of-order and in-order — sharing a common frontend, memory hierarchy, and SoC device layer. It boots Linux 6.6 through OpenSBI to a BusyBox shell and passes all 134/134 riscv-tests.

Install

pip install rvsim

Requires Python 3.10+. Ships pre-built wheels for Linux x86_64.

Quick Start

from rvsim import Config, BranchPredictor, Cache, Environment

config = Config(
    width=4,
    branch_predictor=BranchPredictor.TAGE(),
    l1d=Cache("32KB", ways=8, latency=1, mshr_count=8),
    l2=Cache("256KB", ways=8, latency=10),
)

result = Environment(binary="program.elf", config=config).run()
print(result.stats.query("ipc|branch|miss"))

  ipc                          0.9256
  branch_accuracy_pct         83.3474
  branch_mispredictions       497,026
  dcache_misses                79,488
  l2_misses                    21,374

Features

Two Pipeline Backends

Out-of-order superscalar — Physical register file with dual rename maps (speculative + committed), CAM-style issue queue with wakeup/select and oldest-first priority, reorder buffer for in-order commit with precise exceptions, load queue for memory ordering violation detection, store buffer with forwarding, and a configurable functional unit pool (per-type counts and latencies).

In-order scalar — Scoreboard-based operand tracking, FIFO issue queue with head-of-queue blocking, backpressure gating. Shares the same frontend and commit/memory/writeback stages as the O3 backend, making both modes directly comparable on identical workloads.

Both backends enforce identical serialization semantics: system/CSR instructions wait for all older completions, FENCE respects predecessor/successor ordering bits, loads wait for older store address resolution.

Memory Hierarchy

• SV39 virtual memory — separate iTLB/dTLB, shared L2 TLB, full hardware page table walker with A/D bit management
• L1i / L1d / L2 / L3 caches — independently configurable size, associativity, latency, and replacement policy (LRU, PLRU, FIFO, Random, MRU)
• Non-blocking L1D via MSHRs with request coalescing
• Hardware prefetchers per cache level: next-line, stride, stream, tagged
• Inclusion policies: non-inclusive, inclusive (back-invalidation), exclusive (L1-L2 swap)
• DRAM controller — row-buffer aware timing (tCAS, tRAS, tPRE, row-miss, bank interleaving, refresh)

Branch Prediction

Five pluggable predictors with shared BTB, RAS, and global history register:

Predictor	Description
Static	Always not-taken (baseline)
GShare	PC XOR global history, 2-bit counters
Tournament	Local + global two-level adaptive with meta-predictor
Perceptron	Neural predictor with weight vectors
TAGE	Tagged geometric history length with loop predictor

RAS recognizes both x1 and x5 as link registers per RISC-V spec Table 2.1, including coroutine swap detection.

ISA & Privileged Architecture

RV64IMAFDC — base integer, multiply/divide, atomics (LR/SC + AMO), single/double float with IEEE 754 NaN-boxing, compressed instructions. M/S/U privilege modes, full CSR set, trap delegation, MRET/SRET, WFI, SFENCE.VMA, FENCE/FENCE.I, PMP (16 regions).

Passes all 134/134 tests in riscv-software-src/riscv-tests.

SoC Devices

CLINT timer, PLIC interrupt controller, 16550A UART, VirtIO MMIO block device, Goldfish RTC, SYSCON (poweroff/reboot), HTIF. Auto-generated device tree blob.

Python API

Comparing Configurations

from rvsim import BranchPredictor, Config, Environment, Stats

rows = {}
for name, bp in [("GShare", BranchPredictor.GShare()), ("TAGE", BranchPredictor.TAGE())]:
    r = Environment("program.elf", Config(branch_predictor=bp)).run()
    rows[name] = r.stats

print(Stats.tabulate(rows, title="Branch Predictor Comparison"))

Parallel Sweeps

Sweep distributes all (binary, config) combinations across CPU cores:

from rvsim import Sweep, Config, Cache

results = Sweep(
    binaries=["qsort.elf", "mandelbrot.elf", "maze.elf"],
    configs={
        f"L1={s}": Config(l1d=Cache(s, ways=8, mshr_count=8), uart_quiet=True)
        for s in ["8KB", "16KB", "32KB", "64KB"]
    },
).run(parallel=True)

results.compare(metrics=["ipc", "dcache_misses"], baseline="L1=8KB")

Low-Level Control

from rvsim import Simulator, Config, reg, csr

cpu = Simulator().config(Config(width=4)).binary("program.elf").build()

for _ in range(1000):
    cpu.tick()
    cpu.pipeline_snapshot().visualize()

cpu.run_until(pc=0x80001234)
cpu.run_until(privilege="U")

print(hex(cpu.regs[reg.A0]))
print(hex(cpu.csrs[csr.MSTATUS]))
print(cpu.mem64[0x80001000])

cpu.save("checkpoint.bin")

Analysis Scripts

Ready-to-run design-space exploration in scripts/analysis/:

Script	Description
branch_predict.py	Accuracy comparison across all 5 predictors
cache_sweep.py	L1D size vs miss rate and IPC impact
design_space.py	Multi-dimensional width x cache size sweep
o3_inorder.py	Out-of-order vs in-order backend comparison
width_scaling.py	IPC vs superscalar width
stall_breakdown.py	Stall cycle attribution (memory, control, data)
top_down.py	Top-down microarchitecture analysis
inst_mix.py	Instruction class breakdown

rvsim scripts/analysis/branch_predict.py
rvsim scripts/analysis/cache_sweep.py --sizes 4KB 8KB 16KB 32KB 64KB
rvsim scripts/analysis/o3_inorder.py --widths 1 2 4

Building from Source

Requires Rust (2024 edition), Python 3.10+, and riscv64-unknown-elf-gcc.

git clone https://github.com/willmccallion/rvsim
cd rvsim
python3 -m venv .venv && source .venv/bin/activate
pip install maturin
maturin develop --release
make -C software

Linux Boot

Boots Linux 6.6 through OpenSBI to a BusyBox shell on both backends.

make -C software linux              # Build kernel + rootfs via Buildroot
rvsim scripts/setup/boot_linux.py   # Boot (login: root, no password)

Documentation

Full documentation including architecture deep-dives, API reference, and examples:

willmccallion.github.io/rvsim

License

Licensed under either of MIT or Apache-2.0, at your option.