The cycle and instret counters serve to support user mode self-profiling usages, wherein a user can read the counter(s) twice and compute the delta(s) to evaluate user software performance and behavior. Currently, these counters are not filtered by privilege mode, and thus they continue to increment while traps (e.g., page faults or interrupts) to more privileged code are handled. This causes two problems:
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It introduces unpredictable noise to the counter values observed by the user.
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It leaks information about privileged software execution to user mode.
This proposal remedies these issues by introducing privilege mode filtering for the cycle and instret counters.