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Here, "suitably endowed" means that the System Control Coprocessor (#15) has Performance Monitoring Registers, including a CCNT (Cycle Count) register. The CCNT register is used in a way similar to the TSC register in x86 processors by the get_cyclecount(9) function. The entropy-harvesting thread is a heavy user of this function, and will benefit from not having to call binuptime(9) instead. One problem with the CCNT register is that it is 32-bit only, so the upper 32-bits of the returned number are always 0. The entropy harvester does not care, but in case any one else does, follow-up work may include an interrup trap to increment an upper-32-bit counter on CCNT overflow. Another problem is that the CCNT register is not readable in user-mode code; in can be made readable by userland, but then it is also writable, and so is a good chunk of the PMU system. For that reason, the CCNT is not enabled for user-mode access in this commit. Like the x86, there is one CCNT per core, so they don't all run in perfect sync. Reviewed by: ian@ (an earlier version) Tested by: ian@ (same earlier version) Committed from: WANDBOARD-QUAD
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