ARM皮质A7在核模式下返回PMCCNTR = 0，在用户模式下返回非法指令(即使在PMUSERENR = 1之后)

I want to read the cycle count register (PMCCNTR) on a Raspberry Pi 2, which has an ARM Cortex A7 core. I compile a kernel module for it as follows:I want to read the cycle count register (PMCCNT