Enabling Virtual Memory Research on RISC-V with a Configurable TLB Hierarchy for the Rocket Chip Generator

Abstract

This work enhances the RISC-V Rocket Chip Generator by implementing a configurable, set-associative Translation Lookaside Buffer (TLB) hierarchy within its Memory Management Unit. This innovation lifts the previous restrictions of fixed L1/L2 TLB designs, allowing flexible configurations from direct-mapped to fully-associative structures. The configurable TLBs enable critical virtual memory research aimed at optimizing performance versus resource utilization, validated using SPEC2006 benchmarks on FPGA hardware.

Report

Key Highlights

• Innovation Target: The Memory Management Unit (MMU) and TLB hierarchy of the open-source RISC-V Rocket Chip Generator.
• Core Feature: Implementation of configurable, set-associative templates for both L1 (Instruction/Data) and shared L2 TLBs.
• Restriction Lifted: Replaces the original fixed TLB configurations (L1 fully-associative, L2 direct-mapped) with flexible designs.
• Research Enabler: Allows architects to create any TLB organization required, ranging from direct-mapped to fully-associative structures.
• Validation: Evaluation of different TLB configurations was performed using the SPEC2006 benchmark suite.
• Results Measured: Performance, area utilization, and operating frequency results were collected.

Technical Details

• Base Platform: Rocket Chip Generator, a powerful tool for producing parameterized RISC-V-based SoCs.
• Component Focus: Translation Lookaside Buffers (TLBs), which mitigate the performance overhead of frequent Page Table Walks.
• New Architecture: The configurable TLB templates utilize set-associativity, providing flexibility in cache design (set count and ways).
• Design Goal: To find the optimal balance (ratio) between performance gains and resource consumption (e.g., size and impact on the critical path).
• Hardware Used for Evaluation: The Xilinx ZCU102 FPGA platform was utilized to test the implemented designs.
• Measurements: The reported results quantify the trade-offs in terms of performance (speed/IPC), silicon area (resource utilization), and maximum clock frequency (critical path impact).

Implications

• Advancing RISC-V Research: By making the crucial TLB hierarchy fully customizable, this work significantly enhances the utility of the Rocket Chip Generator for academic and industrial virtual memory research.
• Optimization Potential: Allows chip designers to precisely tune the TLB parameters for specific applications or target environments (e.g., high-performance computing versus resource-constrained embedded systems).
• Standardizing Flexibility: Provides a standardized, parameterized methodology for studying MMU performance bottlenecks and architectural choices within the leading open-source RISC-V ecosystem.
• Performance Insight: Directly facilitates experiments proving how different TLB sizes and associativities affect overall processor speed and efficiency when running real-world workloads like SPEC2006.