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vclock_gettime.c

/*
 * 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")));

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