Evaluating IOMMU-Based Shared Virtual Addressing for RISC-V Embedded Heterogeneous SoCs

Embedded heterogeneous systems-on-chip (SoCs) rely on domain-specific hardware accelerators to improve performance and energy efficiency. In particular, programmable multi-core accelerators feature a cluster of processing elements and tightly coupled scratchpad memories to balance performance, energy efficiency, and flexibility. In embedded systems running a general-purpose OS, accelerators access data via dedicated, physically addressed memory regions. This negatively impacts memory utilization and performance by requiring a copy from the virtual host address to the physical accelerator address space. Input-Output Memory Management Units (IOMMUs) overcome this limitation by allowing devices and hosts to use a shared virtual paged address space. However, resolving IO virtual addresses can be particularly costly on high-latency memory systems as it requires up to three sequential memory accesses on IOTLB miss. In this work, we present a quantitative evaluation of shared virtual addressing in RISC-V heterogeneous embedded systems. We integrate an IOMMU in an open source heterogeneous RISC-V SoC consisting of a 64-bit host with a 32-bit accelerator cluster. We evaluated the system performance by emulating the design on FPGA and implementing compute kernels from the RajaPERF benchmark suite using heterogeneous OpenMP programming. We measure the transfers and computation time on the host and accelerators for systems with different DRAM access latencies. We first show that IO virtual address translation can account for 4.2% up to 17.6% of the accelerator's runtime for gemm (General Matrix Multiplication) at low and high memory bandwidth. Then, we show that in systems containing a last-level cache, this IO address translation cost falls to 0.4% and 0.7% under the same conditions, making shared-virtual addressing and zero-copy offloading suitable for such RISC-V heterogeneous SoCs.