Fast TLB Simulation for RISC-V Systems

Abstract

This paper introduces a novel, fast TLB simulation framework specifically designed for exploring Translation Lookaside Buffer (TLB) behaviors in multi-core RISC-V systems. Integrated into the QEMU dynamic binary translated emulator, the framework allows for rapid, flexible prototyping and profiling of various TLB hardware designs. Demonstrating high efficiency, the simulator achieves 400 MIPS on an 8-core system with minimal performance overhead, enabling the exploration of instruction-set architecture extensions.

Report

Structured Report: Fast TLB Simulation for RISC-V Systems

Key Highlights

• High-Speed Simulation: The TLB simulation framework achieves high performance, running at approximately 400 Million Instructions Per Second (MIPS) when simulating complicated multi-level shared TLB designs in an 8-core system.
• Low Overhead: The simulation incurs only an 18% performance overhead compared to unmodified QEMU, based on benchmarks exhibiting a 1% L1 TLB miss rate.
• Flexibility and Prototyping: The tool allows designers to rapidly prototype, validate, profile, and benchmark various hardware TLB design choices permitted by the flexible RISC-V privileged specification.
• ISA Extension Proposal: The simulation results were utilized to explore instruction-set level design space, specifically testing designs currently unauthorized by the specification, leading to a proposal for extending RISC-V's virtual memory system.

Technical Details

• Target System: Multi-core RISC-V systems (addressing the RISC-V privileged specification flexibility).
• Methodology: Online TLB simulation is achieved by integrating the framework with the dynamic binary translated emulator QEMU.
• Performance Metrics: The framework was tested simulating complex, multi-level shared TLB designs.
• Testing Scope: The tool demonstrated its utility by simulating a shared last-level TLB design, which is currently not permitted under the existing RISC-V privileged specification, to gather empirical data.

Implications

• Accelerating RISC-V Hardware Design: This fast and versatile simulator is a critical asset for hardware architects, allowing them to efficiently prototype and evaluate different TLB organizations—a key component of memory management—before committing to silicon.
• Driving Standards Evolution: By enabling the testing of designs that violate current specifications, the framework provides necessary empirical evidence to propose and justify data-driven extensions to the RISC-V virtual memory and privileged specification.
• Enhancing Software Quality: The ability to profile and benchmark software against precise TLB behaviors aids operating system developers and application designers in optimizing code for improved performance and resource utilization across diverse RISC-V platforms.