vclock_gettime.c

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/*
 * Copyright 2006 Andi Kleen, SUSE Labs.
 * Subject to the GNU Public License, v.2
 *
 * Fast user context implementation of clock_gettime, gettimeofday, and time.
 *
 * The code should have no internal unresolved relocations.
 * Check with readelf after changing.
 */

/* Disable profiling for userspace code: */
#define DISABLE_BRANCH_PROFILING

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

#define gtod (&VVAR(vsyscall_gtod_data))

notrace static cycle_t vread_tsc(void)
{
    cycle_t ret;
    u64 last;

    /*
     * Empirically, a fence (of type that depends on the CPU)
     * before rdtsc is enough to ensure that rdtsc is ordered
     * with respect to loads.  The various CPU manuals are unclear
     * as to whether rdtsc can be reordered with later loads,
     * but no one has ever seen it happen.
     */
    rdtsc_barrier();
    ret = (cycle_t)vget_cycles();

    last = VVAR(vsyscall_gtod_data).clock.cycle_last;

    if (likely(ret >= last))
        return ret;

    /*
     * GCC likes to generate cmov here, but this branch is extremely
     * predictable (it's just a funciton of time and the likely is
     * very likely) and there's a data dependence, so force GCC
     * to generate a branch instead.  I don't barrier() because
     * we don't actually need a barrier, and if this function
     * ever gets inlined it will generate worse code.
     */
    asm volatile ("");
    return last;
}

static notrace cycle_t vread_hpet(void)
{
    return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
}

notrace 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;
}

notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
{
    long ret;

    asm("syscall" : "=a" (ret) :
        "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
    return ret;
}


notrace static inline u64 vgetsns(void)
{
    long v;
    cycles_t cycles;
    if (gtod->clock.vclock_mode == VCLOCK_TSC)
        cycles = vread_tsc();
    else if (gtod->clock.vclock_mode == VCLOCK_HPET)
        cycles = vread_hpet();
    else
        return 0;
    v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask;
    return v * gtod->clock.mult;
}

/* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
notrace static int __always_inline do_realtime(struct timespec *ts)
{
    unsigned long seq;
    u64 ns;
    int mode;

    ts->tv_nsec = 0;
    do {
        seq = read_seqcount_begin(&gtod->seq);
        mode = gtod->clock.vclock_mode;
        ts->tv_sec = gtod->wall_time_sec;
        ns = gtod->wall_time_snsec;
        ns += vgetsns();
        ns >>= gtod->clock.shift;
    } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));

    timespec_add_ns(ts, ns);
    return mode;
}

notrace static int do_monotonic(struct timespec *ts)
{
    unsigned long seq;
    u64 ns;
    int mode;

    ts->tv_nsec = 0;
    do {
        seq = read_seqcount_begin(&gtod->seq);
        mode = gtod->clock.vclock_mode;
        ts->tv_sec = gtod->monotonic_time_sec;
        ns = gtod->monotonic_time_snsec;
        ns += vgetsns();
        ns >>= gtod->clock.shift;
    } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
    timespec_add_ns(ts, ns);

    return mode;
}

notrace static int do_realtime_coarse(struct timespec *ts)
{
    unsigned long seq;
    do {
        seq = read_seqcount_begin(&gtod->seq);
        ts->tv_sec = gtod->wall_time_coarse.tv_sec;
        ts->tv_nsec = gtod->wall_time_coarse.tv_nsec;
    } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
    return 0;
}

notrace static int do_monotonic_coarse(struct timespec *ts)
{
    unsigned long seq;
    do {
        seq = read_seqcount_begin(&gtod->seq);
        ts->tv_sec = gtod->monotonic_time_coarse.tv_sec;
        ts->tv_nsec = gtod->monotonic_time_coarse.tv_nsec;
    } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));

    return 0;
}

notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
{
    int ret = VCLOCK_NONE;

    switch (clock) {
    case CLOCK_REALTIME:
        ret = do_realtime(ts);
        break;
    case CLOCK_MONOTONIC:
        ret = do_monotonic(ts);
        break;
    case CLOCK_REALTIME_COARSE:
        return do_realtime_coarse(ts);
    case CLOCK_MONOTONIC_COARSE:
        return do_monotonic_coarse(ts);
    }

    if (ret == VCLOCK_NONE)
        return vdso_fallback_gettime(clock, ts);
    return 0;
}
int clock_gettime(clockid_t, struct timespec *)
    __attribute__((weak, alias("__vdso_clock_gettime")));

notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
{
    long ret = VCLOCK_NONE;

    if (likely(tv != NULL)) {
        BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
                 offsetof(struct timespec, tv_nsec) ||
                 sizeof(*tv) != sizeof(struct timespec));
        ret = do_realtime((struct timespec *)tv);
        tv->tv_usec /= 1000;
    }
    if (unlikely(tz != NULL)) {
        /* Avoid memcpy. Some old compilers fail to inline it */
        tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
        tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
    }

    if (ret == VCLOCK_NONE)
        return vdso_fallback_gtod(tv, tz);
    return 0;
}
int gettimeofday(struct timeval *, struct timezone *)
    __attribute__((weak, alias("__vdso_gettimeofday")));

/*
 * This will break when the xtime seconds get inaccurate, but that is
 * unlikely
 */
notrace time_t __vdso_time(time_t *t)
{
    /* This is atomic on x86_64 so we don't need any locks. */
    time_t result = ACCESS_ONCE(VVAR(vsyscall_gtod_data).wall_time_sec);

    if (t)
        *t = result;
    return result;
}
int time(time_t *t)
    __attribute__((weak, alias("__vdso_time")));