Nile: A Programmable Monitoring Coprocessor

Researchers widely employ hardware performance counters (HPCs) as well as debugging and profiling tools in processors for monitoring different events such as cache hits, cache misses, and branch prediction statistics during the execution of programs. The collected information can be used for power, performance, and thermal management of the system as well as detecting anomalies or malicious behavior in the software. However, monitoring new or complex events using HPCs and existing tools is a challenging task because HPCs only provide a fixed pool of raw events to monitor. To address this challenge, we propose the implementation of a programmable hardware monitor in a complete system framework including the hardware monitor architecture and its interface with an in-order single-issue RISC-V processor as well as an operating system. As a proof of concept, we demonstrate how to programmatically implement a shadow stack using our hardware monitor and how the programmed shadow stack detects stack buffer overflow attacks. Our hardware monitor design incurs a 26 percent power overhead and a 15 percent area overhead over an unmodified RISC-V processor. Our programmed shadow stack has less than 3 percent performance overhead in the worst case.