vclock_gettime.c

linux 内核源代码

/*
 * Copyright 2006 Andi Kleen, SUSE Labs.
 * Subject to the GNU Public License, v.2
 *
 * Fast user context implementation of clock_gettime and gettimeofday.
 *
 * The code should have no internal unresolved relocations.
 * Check with readelf after changing.
 * Also alternative() doesn't work.
 */

#include <linux/kernel.h>
#include <linux/posix-timers.h>
#include <linux/time.h>
#include <linux/string.h>
#include <asm/vsyscall.h>
#include <asm/vgtod.h>
#include <asm/timex.h>
#include <asm/hpet.h>
#include <asm/unistd.h>
#include <asm/io.h>
#include <asm/vgtod.h>
#include "vextern.h"

#define gtod vdso_vsyscall_gtod_data

static long vdso_fallback_gettime(long clock, struct timespec *ts)
{
	long ret;
	asm("syscall" : "=a" (ret) :
	    "0" (__NR_clock_gettime),"D" (clock), "S" (ts) : "memory");
	return ret;
}

static inline long vgetns(void)
{
	long v;
	cycles_t (*vread)(void);
	vread = gtod->clock.vread;
	v = (vread() - gtod->clock.cycle_last) & gtod->clock.mask;
	return (v * gtod->clock.mult) >> gtod->clock.shift;
}

static noinline int do_realtime(struct timespec *ts)
{
	unsigned long seq, ns;
	do {
		seq = read_seqbegin(&gtod->lock);
		ts->tv_sec = gtod->wall_time_sec;
		ts->tv_nsec = gtod->wall_time_nsec;
		ns = vgetns();
	} while (unlikely(read_seqretry(&gtod->lock, seq)));
	timespec_add_ns(ts, ns);
	return 0;
}

/* Copy of the version in kernel/time.c which we cannot directly access */
static void vset_normalized_timespec(struct timespec *ts, long sec, long nsec)
{
	while (nsec >= NSEC_PER_SEC) {
		nsec -= NSEC_PER_SEC;
		++sec;
	}
	while (nsec < 0) {
		nsec += NSEC_PER_SEC;
		--sec;
	}
	ts->tv_sec = sec;
	ts->tv_nsec = nsec;
}

static noinline int do_monotonic(struct timespec *ts)
{
	unsigned long seq, ns, secs;
	do {
		seq = read_seqbegin(&gtod->lock);
		secs = gtod->wall_time_sec;
		ns = gtod->wall_time_nsec + vgetns();
		secs += gtod->wall_to_monotonic.tv_sec;
		ns += gtod->wall_to_monotonic.tv_nsec;
	} while (unlikely(read_seqretry(&gtod->lock, seq)));
	vset_normalized_timespec(ts, secs, ns);
	return 0;
}

int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
{
	if (likely(gtod->sysctl_enabled && gtod->clock.vread))
		switch (clock) {
		case CLOCK_REALTIME:
			return do_realtime(ts);
		case CLOCK_MONOTONIC:
			return do_monotonic(ts);
		}
	return vdso_fallback_gettime(clock, ts);
}
int clock_gettime(clockid_t, struct timespec *)
	__attribute__((weak, alias("__vdso_clock_gettime")));

int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
{
	long ret;
	if (likely(gtod->sysctl_enabled && gtod->clock.vread)) {
		BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
			     offsetof(struct timespec, tv_nsec) ||
			     sizeof(*tv) != sizeof(struct timespec));
		do_realtime((struct timespec *)tv);
		tv->tv_usec /= 1000;
		if (unlikely(tz != NULL)) {
			/* This relies on gcc inlining the memcpy. We'll notice
			   if it ever fails to do so. */
			memcpy(tz, &gtod->sys_tz, sizeof(struct timezone));
		}
		return 0;
	}
	asm("syscall" : "=a" (ret) :
	    "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
	return ret;
}
int gettimeofday(struct timeval *, struct timezone *)
	__attribute__((weak, alias("__vdso_gettimeofday")));