Design, Implementation and Evaluation of the SVNAPOT Extension on a RISC-V Processor

Abstract

The paper details the design, implementation, and evaluation of the RISC-V SVNAPOT Extension, intended to reduce Memory Management Unit (MMU) overhead during heavy memory loads. This extension leverages larger 64KB Natural-Power-of-Two (NAPOT) pages alongside standard 4KB pages. The authors successfully extended the Rocket Chip Generator MMU to manage this page size collocation within the L2 TLB and presented a sensitivity analysis of their optimized design.

Report

Key Highlights

• Performance Remediation: The core goal is to remedy the performance overhead associated with the Memory Management Unit (MMU), particularly under conditions of heavy memory loads.
• SVA N A P O T Extension: The work focuses entirely on the design and evaluation of the RISC-V SVNAPOT Extension, a privileged specification feature.
• NAPOT Pages: The implementation utilizes Natural-Power-of-Two (NAPOT) multiples of the standard 4KB page size, with 64KB identified as the default candidate.
• Rocket Chip Integration: The extension was implemented by modifying and extending the MMU of the popular open-source Rocket Chip Generator framework.
• L2 TLB Management: A significant design challenge was managing the collocation (coexistence) of both 4KB base pages and the larger 64KB NAPOT pages within the L2 Translation Lookaside Buffer (TLB).

Technical Details

• Target Standard: RISC-V Privileged Specification defining the SVNAPOT Extension.
• Base Page Size: 4KB.
• Extended Page Size: 64KB (as the chosen NAPOT candidate).
• Implementation Platform: MMU within the Rocket Chip Generator architecture.
• Key Design Challenge: Enabling the L2 TLB to efficiently handle entries of different sizes (4KB and 64KB) simultaneously.
• Evaluation Method: Preliminary sensitivity analysis was conducted on the L2 TLB using different configurations and page size mixes to assess performance trade-offs.

Implications

• Ecosystem Performance Boost: By addressing MMU performance overhead, the SVNAPOT extension significantly improves the efficiency and speed of RISC-V systems running memory-intensive applications.
• Standard Implementation Validation: The paper provides a concrete implementation and evaluation of a proposed RISC-V privileged specification feature, aiding its adoption and standardization across the ecosystem.
• Hardware Design Guidance: The analysis of design challenges and trade-offs related to 4KB/64KB collocation in the L2 TLB offers critical guidance for future high-performance RISC-V processor and MMU designs.
• Foundation for Future Work: The research summarizes techniques that could further enhance memory management performance, contributing to ongoing optimization efforts in RISC-V hardware architecture.