A Nice Kernel Trick for x86

Today I learnt a nice way to obtain the number of microseconds since the power-up of the CPU using a P5 performance counter via (rdtsc). This instruction returns the number of ticks since power-up but it can be scaled to microseconds by dividing the number by the CPU frequency (in MHz).

The kernel code reads:

#include <linux/kernel.h>
#include <linux/cpufreq.h>

unsigned long long uSecSinceBoot()
{
  volatile unsigned long long int cpu_ticks;

  __asm__("rdtsc\n\t"
          "mov %%edx, %%ecx\n\t"
          :"=A" (cpu_ticks));

  unsigned long long div = cpu_ticks * 1000;
  do_div(div, cpu_khz); // TRICK! div is modified, do_div() returns reminder

  return div;
}

I am using do_div() as on x86/32 bits long-long division is supported by GCC via an external function (_udivdi3) which usually lives in libgcc but it is not provided by the kernel.

-ulianov

P.S. I did extract _udivdi3 from libgcc and then disassembled/reassembled it but it's a pain. do_div is the right thing to use in the kernel.