linux/include/asm-x86/tsc.h

/*
 * x86 TSC related functions
 */
#ifndef _ASM_X86_TSC_H
#define _ASM_X86_TSC_H

#include <asm/processor.h>

#define NS_SCALE	10 /* 2^10, carefully chosen */
#define US_SCALE	32 /* 2^32, arbitralrily chosen */

/*
 * Standard way to access the cycle counter.
 */
typedef unsigned long long cycles_t;

extern unsigned int cpu_khz;
extern unsigned int tsc_khz;

static inline cycles_t get_cycles(void)
{
	unsigned long long ret = 0;

#ifndef CONFIG_X86_TSC
	if (!cpu_has_tsc)
		return 0;
#endif

#if defined(CONFIG_X86_GENERIC) || defined(CONFIG_X86_TSC)
	rdtscll(ret);
#endif
	return ret;
}

/* Like get_cycles, but make sure the CPU is synchronized. */
static __always_inline cycles_t get_cycles_sync(void)
{
	unsigned long long ret;
	unsigned eax, edx;

	/*
  	 * Use RDTSCP if possible; it is guaranteed to be synchronous
 	 * and doesn't cause a VMEXIT on Hypervisors
	 */
	alternative_io(ASM_NOP3, ".byte 0x0f,0x01,0xf9", X86_FEATURE_RDTSCP,
		       ASM_OUTPUT2("=a" (eax), "=d" (edx)),
		       "a" (0U), "d" (0U) : "ecx", "memory");
	ret = (((unsigned long long)edx) << 32) | ((unsigned long long)eax);
	if (ret)
		return ret;

	/*
	 * Don't do an additional sync on CPUs where we know
	 * RDTSC is already synchronous:
	 */
	alternative_io("cpuid", ASM_NOP2, X86_FEATURE_SYNC_RDTSC,
			  "=a" (eax), "0" (1) : "ebx","ecx","edx","memory");
	rdtscll(ret);

	return ret;
}

extern void tsc_init(void);
extern void mark_tsc_unstable(char *reason);
extern int unsynchronized_tsc(void);
extern void init_tsc_clocksource(void);
int check_tsc_unstable(void);

/*
 * Boot-time check whether the TSCs are synchronized across
 * all CPUs/cores:
 */
extern void check_tsc_sync_source(int cpu);
extern void check_tsc_sync_target(void);

#ifdef CONFIG_X86_64
extern void tsc_calibrate(void);
#endif

#endif