4

I'm trying to get timestamp counter (TSC) of CPU. I've succeeded on my PC with Intel i7 CPU. Assembly code in this links helped me.

Now, I want to do it on my Raspberry Pi model B. The problem is ARmv6 has a different instruction set from Intel CPU. And user-mode prvents me from using some instructions. So I've searched some article to solve it.

But it didnt' work well as I expected. When I run the following two codes (with sudo privilege):

int main(void){
//      printf("try to enable user mode access to performance counter\n");
        asm("MCR p15, 0, %0, C9, C14, 0\n\t" :: "r"(1));
//      printf("disable counter overflow interrupts (just in case)\n");
        asm("MCR p15, 0, %0, C9, C14, 2\n\t" :: "r"(0x8000000f));

        return 0;
}

and

#include <stdio.h>

static inline unsigned int get_cyclecount(void) {
        unsigned int value;
        // Read CCNT Register
        asm volatile ("MRC p15, 0, %0, c9, c13, 0\t\n": "=r"(value));
        return value;
}

int main(void){
        unsigned int t = get_cyclecount();
        printf("%d\n", t);
        return 0;
}

It prints nothing even there is printf

3

I do not believe you can successfully execute those instructions from Linux userland. I thought only Linux kernel code could successfully execute those instructions (sudo/root is not the same as kernel).

A kernel module to enable access to the CPU cycle timer is given at http://blog.regehr.org/archives/794

It uses the following (kernel) code to allow access to the CPU cycle timer from userland.

  asm volatile ("mcr p15,  0, %0, c15,  c9, 0\n" : : "r" (1));
1

There are two ways to solves.

A: You can write a kernel module to enable PMU on hardware. Then read counters from userland.

B: use syscall: syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0); to access related PMU events.

Please check below link for detail.

http://zhiyisun.github.io/2016/03/02/How-to-Use-Performance-Monitor-Unit-(PMU)-of-64-bit-ARMv8-A-in-Linux.html

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