"Interrupting" the Status Quo: A First Glance at the RISC-V Advanced Interrupt Architecture (AIA).

Interrupt controllers are a crucial component in computing platforms. From cloud computing to embedded systems, interrupts and respective controllers enable more efficient management and operation of a platform’s resources. Modern computer architectures incorporate hardware interrupt controllers (e.g., Arm Generic Interrupt Controller (GIC)) that are well-established in modern processors and system-on-chips (SoCs). This article describes our work and research on developing the first open-source RISC-V Advanced Interrupt Architecture (AIA) IP compliant with the recently ratified specification (v1.0). Our contribution is multifold and encompasses architecture, microarchitecture, and evaluation. In particular, we explored alternative designs and microarchitectural enhancements for the implemented IP to cope with mixed-criticality systems (MCS) requirements (e.g., real-time and predictability). From this exploration, we highlight the proposed Integrated Embedded AIA (IE-AIA) design. For each configuration, we assess the impact on hardware utilization and interrupt latency. Due to the increased proliferation of virtualization in MCS, we measured the interrupt latency for a system configuration built atop the Bao hypervisor. At the macro level (i.e., considering both hardware and software), and in particular for a full-blown virtualization stack, we observed a reduction of ~99.5% in the average interrupt latency when comparing the Platform-Level Interrupt Controller (PLIC) IP (full trap and emulation) with the standard AIA IP (no hypervisor mediation due to IMSIC direct interrupt injection). For the IE-AIA, our evaluation focused on the micro view (i.e., hardware only), where we observed that under interference, the IE-AIA IP shows a reduction of $\sim 7\times $ in the average interrupt latency and deterministic behavior compared to the standard AIA implementation. We also provide the first empirical-based comparison between the RISC-V PLIC and the RISC-V AIA. Finally, all artifacts described in this article are open source to foster collaboration and further explore additional design configurations.