Lazy Shadow Paging Under the RISC-V Architecture

Memory virtualization, a core component of virtualization technology, directly impacts the overall performance of virtual machines. Current memory virtualization approaches often involve a tradeoff between the overhead of two-dimensional address translation and page table synchronization. Traditional shadow paging employs an additional software-maintained page table to achieve address translation performance comparable to native systems. However, synchronization of shadow page tables relies on write protection, frequently causing VM-exits that significantly degrade system performance. In contrast, the nested paging approach leverages hardware-assisted virtualization, allowing the guest page table and nested page table to be directly loaded into the MMU. While this eliminates page table synchronization, the two-dimensional page table traversal will seriously degrade the address translation performance. Two-dimensional page table traversal incurs substantial performance penalties for address translation due to privilege overhead. This paper proposes lazy shadow paging (LSP), which reduces page table synchronization overhead while retaining the high efficiency of shadow page tables. Leveraging the privilege model and hardware features of the RISC-V architecture, LSP analyzes the access patterns of guest OS page tables and binds synchronization with translation lookaside buffer (TLB) flushes, reducing the software overhead associated with page table updates by deferring costs until the first access to a relevant page to minimize VM-exits. In addition, it introduces a fast path for handling VM-exits, exploiting the fine-grained TLB interception and privilege-level features of RISC-V to further optimize performance. Experimental results demonstrate that under the baseline RISC-V architecture, LSP reduces VM-exits by up to 50% compared to traditional shadow paging in micro-benchmark tests. For typical applications in the SPEC2006 benchmark suite, LSP reduces VM-exits by up to 25% compared to traditional shadow paging and decreases memory accesses per TLB miss by 12 compared to nested paging.