ARM processors support various performance monitoring registers, the most basic being a cycle count register. This is how to make use of it on the Raspberry Pi 3 with its ARM Cortex-A53 processor. The A53 implements the ARMv8 architecture which can operate in both 64- and 32-bit modes, the Pi 3 uses the 32-bit AArch32 mode, which is more or less backwards compatible with the ARMv7-A architecture, as implemented for example by the Cortex-A7 (used in the early Pi 2’s) and Cortex-A8. I hope I’ve got that right, all these names are confusing
The performance counters are made available through certain coprocessor registers and the mrc and mcr instructions. By default, use of these instructions is only possible in “privileged” mode, ie. from the kernel, so the first thing we need to do is to enable register access from userspace. This can be done through a simple kernel module that can also set up the cycle counter parameters needed (we could do this from userspace after the kernel module has enabled access, but it’s simpler to do everything at once).
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