Microprocessor Verification via Feedback-Adjusted Markov Models

The challenge of verifying a modern microprocessor design is an overwhelming one: Increasingly complex microarchitectures combined with heavy time-to-market pressure have forced microprocessor vendors to employ immense verification teams in the hope of finding the most critical bugs in a timely manner. Unfortunately, too often, size does not seem to matter in verification, as design schedules continue to slip and microprocessors find their way to the marketplace with design errors. In this paper, we describe a novel closed-loop simulation-based approach to hardware verification and present a tool called StressTest that uses our methods to locate hard-to-find corner-case design bugs and performance problems. StressTest is based on a Markov-model-driven random instruction generator with activity monitors. The model is generated from the user-specified template files and is used to generate the instructions sent to the design under test (DUT). In addition, the user specifies key activity nodes within the design that should be stressed and monitored throughout the simulation. The StressTest engine then uses closed-loop feedback techniques to transform the Markov model into one that effectively stresses the user-selected points of interest. In parallel, StressTest monitors the correctness of the DUT response and, if the design behaves against expectation, it reports a bug and a trace leading to it. Using two microarchitectures as example testbeds, we demonstrate that StressTest finds more bugs with less effort than open-loop random instruction test generation techniques