FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_tc.c

     /*-
      * SPDX-License-Identifier: Beerware
      *
      * ----------------------------------------------------------------------------
      * "THE BEER-WARE LICENSE" (Revision 42):
      * <phk@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
      * can do whatever you want with this stuff. If we meet some day, and you think
      * this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
      * ----------------------------------------------------------------------------
     *
     * Copyright (c) 2011, 2015, 2016 The FreeBSD Foundation
     *
     * Portions of this software were developed by Julien Ridoux at the University
     * of Melbourne under sponsorship from the FreeBSD Foundation.
     *
     * Portions of this software were developed by Konstantin Belousov
     * under sponsorship from the FreeBSD Foundation.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_ntp.h"
    #include "opt_ffclock.h"
    
    #include <sys/param.h>
    #include <sys/kernel.h>
    #include <sys/limits.h>
    #include <sys/lock.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/sbuf.h>
    #include <sys/sleepqueue.h>
    #include <sys/sysctl.h>
    #include <sys/syslog.h>
    #include <sys/systm.h>
    #include <sys/timeffc.h>
    #include <sys/timepps.h>
    #include <sys/timetc.h>
    #include <sys/timex.h>
    #include <sys/vdso.h>
    
    /*
     * A large step happens on boot.  This constant detects such steps.
     * It is relatively small so that ntp_update_second gets called enough
     * in the typical 'missed a couple of seconds' case, but doesn't loop
     * forever when the time step is large.
     */
    #define LARGE_STEP      200
    
    /*
     * Implement a dummy timecounter which we can use until we get a real one
     * in the air.  This allows the console and other early stuff to use
     * time services.
     */
    
    static u_int
    dummy_get_timecount(struct timecounter *tc)
    {
            static u_int now;
    
            return (++now);
    }
    
    static struct timecounter dummy_timecounter = {
            dummy_get_timecount, 0, ~0u, 1000000, "dummy", -1000000
    };
    
    struct timehands {
            /* These fields must be initialized by the driver. */
            struct timecounter      *th_counter;
            int64_t                 th_adjustment;
            uint64_t                th_scale;
            u_int                   th_large_delta;
            u_int                   th_offset_count;
            struct bintime          th_offset;
            struct bintime          th_bintime;
            struct timeval          th_microtime;
            struct timespec         th_nanotime;
            struct bintime          th_boottime;
            /* Fields not to be copied in tc_windup start with th_generation. */
            u_int                   th_generation;
            struct timehands        *th_next;
    };
    
    static struct timehands ths[16] = {
        [0] =  {
            .th_counter = &dummy_timecounter,
            .th_scale = (uint64_t)-1 / 1000000,
            .th_large_delta = 1000000,
            .th_offset = { .sec = 1 },
            .th_generation = 1,
        },
    };
    
    static struct timehands *volatile timehands = &ths[0];
    struct timecounter *timecounter = &dummy_timecounter;
    static struct timecounter *timecounters = &dummy_timecounter;
    
   /* Mutex to protect the timecounter list. */
   static struct mtx tc_lock;
   
   int tc_min_ticktock_freq = 1;
   
   volatile time_t time_second = 1;
   volatile time_t time_uptime = 1;
   
   /*
    * The system time is always computed by summing the estimated boot time and the
    * system uptime. The timehands track boot time, but it changes when the system
    * time is set by the user, stepped by ntpd or adjusted when resuming. It
    * is set to new_time - uptime.
    */
   static int sysctl_kern_boottime(SYSCTL_HANDLER_ARGS);
   SYSCTL_PROC(_kern, KERN_BOOTTIME, boottime,
       CTLTYPE_STRUCT | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
       sysctl_kern_boottime, "S,timeval",
       "Estimated system boottime");
   
   SYSCTL_NODE(_kern, OID_AUTO, timecounter, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
       "");
   static SYSCTL_NODE(_kern_timecounter, OID_AUTO, tc,
       CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
       "");
   
   static int timestepwarnings;
   SYSCTL_INT(_kern_timecounter, OID_AUTO, stepwarnings, CTLFLAG_RWTUN,
       &timestepwarnings, 0, "Log time steps");
   
   static int timehands_count = 2;
   SYSCTL_INT(_kern_timecounter, OID_AUTO, timehands_count,
       CTLFLAG_RDTUN | CTLFLAG_NOFETCH,
       &timehands_count, 0, "Count of timehands in rotation");
   
   struct bintime bt_timethreshold;
   struct bintime bt_tickthreshold;
   sbintime_t sbt_timethreshold;
   sbintime_t sbt_tickthreshold;
   struct bintime tc_tick_bt;
   sbintime_t tc_tick_sbt;
   int tc_precexp;
   int tc_timepercentage = TC_DEFAULTPERC;
   static int sysctl_kern_timecounter_adjprecision(SYSCTL_HANDLER_ARGS);
   SYSCTL_PROC(_kern_timecounter, OID_AUTO, alloweddeviation,
       CTLTYPE_INT | CTLFLAG_RWTUN | CTLFLAG_MPSAFE, 0, 0,
       sysctl_kern_timecounter_adjprecision, "I",
       "Allowed time interval deviation in percents");
   
   volatile int rtc_generation = 1;
   
   static int tc_chosen;   /* Non-zero if a specific tc was chosen via sysctl. */
   static char tc_from_tunable[16];
   
   static void tc_windup(struct bintime *new_boottimebin);
   static void cpu_tick_calibrate(int);
   
   void dtrace_getnanotime(struct timespec *tsp);
   void dtrace_getnanouptime(struct timespec *tsp);
   
   static int
   sysctl_kern_boottime(SYSCTL_HANDLER_ARGS)
   {
           struct timeval boottime;
   
           getboottime(&boottime);
   
   /* i386 is the only arch which uses a 32bits time_t */
   #ifdef __amd64__
   #ifdef SCTL_MASK32
           int tv[2];
   
           if (req->flags & SCTL_MASK32) {
                   tv[0] = boottime.tv_sec;
                   tv[1] = boottime.tv_usec;
                   return (SYSCTL_OUT(req, tv, sizeof(tv)));
           }
   #endif
   #endif
           return (SYSCTL_OUT(req, &boottime, sizeof(boottime)));
   }
   
   static int
   sysctl_kern_timecounter_get(SYSCTL_HANDLER_ARGS)
   {
           u_int ncount;
           struct timecounter *tc = arg1;
   
           ncount = tc->tc_get_timecount(tc);
           return (sysctl_handle_int(oidp, &ncount, 0, req));
   }
   
   static int
   sysctl_kern_timecounter_freq(SYSCTL_HANDLER_ARGS)
   {
           uint64_t freq;
           struct timecounter *tc = arg1;
   
           freq = tc->tc_frequency;
           return (sysctl_handle_64(oidp, &freq, 0, req));
   }
   
   /*
    * Return the difference between the timehands' counter value now and what
    * was when we copied it to the timehands' offset_count.
    */
   static __inline u_int
   tc_delta(struct timehands *th)
   {
           struct timecounter *tc;
   
           tc = th->th_counter;
           return ((tc->tc_get_timecount(tc) - th->th_offset_count) &
               tc->tc_counter_mask);
   }
   
   static __inline void
   bintime_add_tc_delta(struct bintime *bt, uint64_t scale,
       uint64_t large_delta, uint64_t delta)
   {
           uint64_t x;
   
           if (__predict_false(delta >= large_delta)) {
                   /* Avoid overflow for scale * delta. */
                   x = (scale >> 32) * delta;
                   bt->sec += x >> 32;
                   bintime_addx(bt, x << 32);
                   bintime_addx(bt, (scale & 0xffffffff) * delta);
           } else {
                   bintime_addx(bt, scale * delta);
           }
   }
   
   /*
    * Functions for reading the time.  We have to loop until we are sure that
    * the timehands that we operated on was not updated under our feet.  See
    * the comment in <sys/time.h> for a description of these 12 functions.
    */
   
   static __inline void
   bintime_off(struct bintime *bt, u_int off)
   {
           struct timehands *th;
           struct bintime *btp;
           uint64_t scale;
           u_int delta, gen, large_delta;
   
           do {
                   th = timehands;
                   gen = atomic_load_acq_int(&th->th_generation);
                   btp = (struct bintime *)((vm_offset_t)th + off);
                   *bt = *btp;
                   scale = th->th_scale;
                   delta = tc_delta(th);
                   large_delta = th->th_large_delta;
                   atomic_thread_fence_acq();
           } while (gen == 0 || gen != th->th_generation);
   
           bintime_add_tc_delta(bt, scale, large_delta, delta);
   }
   #define GETTHBINTIME(dst, member)                                       \
   do {                                                                    \
           _Static_assert(_Generic(((struct timehands *)NULL)->member,     \
               struct bintime: 1, default: 0) == 1,                        \
               "struct timehands member is not of struct bintime type");   \
           bintime_off(dst, __offsetof(struct timehands, member));         \
   } while (0)
   
   static __inline void
   getthmember(void *out, size_t out_size, u_int off)
   {
           struct timehands *th;
           u_int gen;
   
           do {
                   th = timehands;
                   gen = atomic_load_acq_int(&th->th_generation);
                   memcpy(out, (char *)th + off, out_size);
                   atomic_thread_fence_acq();
           } while (gen == 0 || gen != th->th_generation);
   }
   #define GETTHMEMBER(dst, member)                                        \
   do {                                                                    \
           _Static_assert(_Generic(*dst,                                   \
               __typeof(((struct timehands *)NULL)->member): 1,            \
               default: 0) == 1,                                           \
               "*dst and struct timehands member have different types");   \
           getthmember(dst, sizeof(*dst), __offsetof(struct timehands,     \
               member));                                                   \
   } while (0)
   
   #ifdef FFCLOCK
   void
   fbclock_binuptime(struct bintime *bt)
   {
   
           GETTHBINTIME(bt, th_offset);
   }
   
   void
   fbclock_nanouptime(struct timespec *tsp)
   {
           struct bintime bt;
   
           fbclock_binuptime(&bt);
           bintime2timespec(&bt, tsp);
   }
   
   void
   fbclock_microuptime(struct timeval *tvp)
   {
           struct bintime bt;
   
           fbclock_binuptime(&bt);
           bintime2timeval(&bt, tvp);
   }
   
   void
   fbclock_bintime(struct bintime *bt)
   {
   
           GETTHBINTIME(bt, th_bintime);
   }
   
   void
   fbclock_nanotime(struct timespec *tsp)
   {
           struct bintime bt;
   
           fbclock_bintime(&bt);
           bintime2timespec(&bt, tsp);
   }
   
   void
   fbclock_microtime(struct timeval *tvp)
   {
           struct bintime bt;
   
           fbclock_bintime(&bt);
           bintime2timeval(&bt, tvp);
   }
   
   void
   fbclock_getbinuptime(struct bintime *bt)
   {
   
           GETTHMEMBER(bt, th_offset);
   }
   
   void
   fbclock_getnanouptime(struct timespec *tsp)
   {
           struct bintime bt;
   
           GETTHMEMBER(&bt, th_offset);
           bintime2timespec(&bt, tsp);
   }
   
   void
   fbclock_getmicrouptime(struct timeval *tvp)
   {
           struct bintime bt;
   
           GETTHMEMBER(&bt, th_offset);
           bintime2timeval(&bt, tvp);
   }
   
   void
   fbclock_getbintime(struct bintime *bt)
   {
   
           GETTHMEMBER(bt, th_bintime);
   }
   
   void
   fbclock_getnanotime(struct timespec *tsp)
   {
   
           GETTHMEMBER(tsp, th_nanotime);
   }
   
   void
   fbclock_getmicrotime(struct timeval *tvp)
   {
   
           GETTHMEMBER(tvp, th_microtime);
   }
   #else /* !FFCLOCK */
   
   void
   binuptime(struct bintime *bt)
   {
   
           GETTHBINTIME(bt, th_offset);
   }
   
   void
   nanouptime(struct timespec *tsp)
   {
           struct bintime bt;
   
           binuptime(&bt);
           bintime2timespec(&bt, tsp);
   }
   
   void
   microuptime(struct timeval *tvp)
   {
           struct bintime bt;
   
           binuptime(&bt);
           bintime2timeval(&bt, tvp);
   }
   
   void
   bintime(struct bintime *bt)
   {
   
           GETTHBINTIME(bt, th_bintime);
   }
   
   void
   nanotime(struct timespec *tsp)
   {
           struct bintime bt;
   
           bintime(&bt);
           bintime2timespec(&bt, tsp);
   }
   
   void
   microtime(struct timeval *tvp)
   {
           struct bintime bt;
   
           bintime(&bt);
           bintime2timeval(&bt, tvp);
   }
   
   void
   getbinuptime(struct bintime *bt)
   {
   
           GETTHMEMBER(bt, th_offset);
   }
   
   void
   getnanouptime(struct timespec *tsp)
   {
           struct bintime bt;
   
           GETTHMEMBER(&bt, th_offset);
           bintime2timespec(&bt, tsp);
   }
   
   void
   getmicrouptime(struct timeval *tvp)
   {
           struct bintime bt;
   
           GETTHMEMBER(&bt, th_offset);
           bintime2timeval(&bt, tvp);
   }
   
   void
   getbintime(struct bintime *bt)
   {
   
           GETTHMEMBER(bt, th_bintime);
   }
   
   void
   getnanotime(struct timespec *tsp)
   {
   
           GETTHMEMBER(tsp, th_nanotime);
   }
   
   void
   getmicrotime(struct timeval *tvp)
   {
   
           GETTHMEMBER(tvp, th_microtime);
   }
   #endif /* FFCLOCK */
   
   void
   getboottime(struct timeval *boottime)
   {
           struct bintime boottimebin;
   
           getboottimebin(&boottimebin);
           bintime2timeval(&boottimebin, boottime);
   }
   
   void
   getboottimebin(struct bintime *boottimebin)
   {
   
           GETTHMEMBER(boottimebin, th_boottime);
   }
   
   #ifdef FFCLOCK
   /*
    * Support for feed-forward synchronization algorithms. This is heavily inspired
    * by the timehands mechanism but kept independent from it. *_windup() functions
    * have some connection to avoid accessing the timecounter hardware more than
    * necessary.
    */
   
   /* Feed-forward clock estimates kept updated by the synchronization daemon. */
   struct ffclock_estimate ffclock_estimate;
   struct bintime ffclock_boottime;        /* Feed-forward boot time estimate. */
   uint32_t ffclock_status;                /* Feed-forward clock status. */
   int8_t ffclock_updated;                 /* New estimates are available. */
   struct mtx ffclock_mtx;                 /* Mutex on ffclock_estimate. */
   
   struct fftimehands {
           struct ffclock_estimate cest;
           struct bintime          tick_time;
           struct bintime          tick_time_lerp;
           ffcounter               tick_ffcount;
           uint64_t                period_lerp;
           volatile uint8_t        gen;
           struct fftimehands      *next;
   };
   
   #define NUM_ELEMENTS(x) (sizeof(x) / sizeof(*x))
   
   static struct fftimehands ffth[10];
   static struct fftimehands *volatile fftimehands = ffth;
   
   static void
   ffclock_init(void)
   {
           struct fftimehands *cur;
           struct fftimehands *last;
   
           memset(ffth, 0, sizeof(ffth));
   
           last = ffth + NUM_ELEMENTS(ffth) - 1;
           for (cur = ffth; cur < last; cur++)
                   cur->next = cur + 1;
           last->next = ffth;
   
           ffclock_updated = 0;
           ffclock_status = FFCLOCK_STA_UNSYNC;
           mtx_init(&ffclock_mtx, "ffclock lock", NULL, MTX_DEF);
   }
   
   /*
    * Reset the feed-forward clock estimates. Called from inittodr() to get things
    * kick started and uses the timecounter nominal frequency as a first period
    * estimate. Note: this function may be called several time just after boot.
    * Note: this is the only function that sets the value of boot time for the
    * monotonic (i.e. uptime) version of the feed-forward clock.
    */
   void
   ffclock_reset_clock(struct timespec *ts)
   {
           struct timecounter *tc;
           struct ffclock_estimate cest;
   
           tc = timehands->th_counter;
           memset(&cest, 0, sizeof(struct ffclock_estimate));
   
           timespec2bintime(ts, &ffclock_boottime);
           timespec2bintime(ts, &(cest.update_time));
           ffclock_read_counter(&cest.update_ffcount);
           cest.leapsec_next = 0;
           cest.period = ((1ULL << 63) / tc->tc_frequency) << 1;
           cest.errb_abs = 0;
           cest.errb_rate = 0;
           cest.status = FFCLOCK_STA_UNSYNC;
           cest.leapsec_total = 0;
           cest.leapsec = 0;
   
           mtx_lock(&ffclock_mtx);
           bcopy(&cest, &ffclock_estimate, sizeof(struct ffclock_estimate));
           ffclock_updated = INT8_MAX;
           mtx_unlock(&ffclock_mtx);
   
           printf("ffclock reset: %s (%llu Hz), time = %ld.%09lu\n", tc->tc_name,
               (unsigned long long)tc->tc_frequency, (long)ts->tv_sec,
               (unsigned long)ts->tv_nsec);
   }
   
   /*
    * Sub-routine to convert a time interval measured in RAW counter units to time
    * in seconds stored in bintime format.
    * NOTE: bintime_mul requires u_int, but the value of the ffcounter may be
    * larger than the max value of u_int (on 32 bit architecture). Loop to consume
    * extra cycles.
    */
   static void
   ffclock_convert_delta(ffcounter ffdelta, uint64_t period, struct bintime *bt)
   {
           struct bintime bt2;
           ffcounter delta, delta_max;
   
           delta_max = (1ULL << (8 * sizeof(unsigned int))) - 1;
           bintime_clear(bt);
           do {
                   if (ffdelta > delta_max)
                           delta = delta_max;
                   else
                           delta = ffdelta;
                   bt2.sec = 0;
                   bt2.frac = period;
                   bintime_mul(&bt2, (unsigned int)delta);
                   bintime_add(bt, &bt2);
                   ffdelta -= delta;
           } while (ffdelta > 0);
   }
   
   /*
    * Update the fftimehands.
    * Push the tick ffcount and time(s) forward based on current clock estimate.
    * The conversion from ffcounter to bintime relies on the difference clock
    * principle, whose accuracy relies on computing small time intervals. If a new
    * clock estimate has been passed by the synchronisation daemon, make it
    * current, and compute the linear interpolation for monotonic time if needed.
    */
   static void
   ffclock_windup(unsigned int delta)
   {
           struct ffclock_estimate *cest;
           struct fftimehands *ffth;
           struct bintime bt, gap_lerp;
           ffcounter ffdelta;
           uint64_t frac;
           unsigned int polling;
           uint8_t forward_jump, ogen;
   
           /*
            * Pick the next timehand, copy current ffclock estimates and move tick
            * times and counter forward.
            */
           forward_jump = 0;
           ffth = fftimehands->next;
           ogen = ffth->gen;
           ffth->gen = 0;
           cest = &ffth->cest;
           bcopy(&fftimehands->cest, cest, sizeof(struct ffclock_estimate));
           ffdelta = (ffcounter)delta;
           ffth->period_lerp = fftimehands->period_lerp;
   
           ffth->tick_time = fftimehands->tick_time;
           ffclock_convert_delta(ffdelta, cest->period, &bt);
           bintime_add(&ffth->tick_time, &bt);
   
           ffth->tick_time_lerp = fftimehands->tick_time_lerp;
           ffclock_convert_delta(ffdelta, ffth->period_lerp, &bt);
           bintime_add(&ffth->tick_time_lerp, &bt);
   
           ffth->tick_ffcount = fftimehands->tick_ffcount + ffdelta;
   
           /*
            * Assess the status of the clock, if the last update is too old, it is
            * likely the synchronisation daemon is dead and the clock is free
            * running.
            */
           if (ffclock_updated == 0) {
                   ffdelta = ffth->tick_ffcount - cest->update_ffcount;
                   ffclock_convert_delta(ffdelta, cest->period, &bt);
                   if (bt.sec > 2 * FFCLOCK_SKM_SCALE)
                           ffclock_status |= FFCLOCK_STA_UNSYNC;
           }
   
           /*
            * If available, grab updated clock estimates and make them current.
            * Recompute time at this tick using the updated estimates. The clock
            * estimates passed the feed-forward synchronisation daemon may result
            * in time conversion that is not monotonically increasing (just after
            * the update). time_lerp is a particular linear interpolation over the
            * synchronisation algo polling period that ensures monotonicity for the
            * clock ids requesting it.
            */
           if (ffclock_updated > 0) {
                   bcopy(&ffclock_estimate, cest, sizeof(struct ffclock_estimate));
                   ffdelta = ffth->tick_ffcount - cest->update_ffcount;
                   ffth->tick_time = cest->update_time;
                   ffclock_convert_delta(ffdelta, cest->period, &bt);
                   bintime_add(&ffth->tick_time, &bt);
   
                   /* ffclock_reset sets ffclock_updated to INT8_MAX */
                   if (ffclock_updated == INT8_MAX)
                           ffth->tick_time_lerp = ffth->tick_time;
   
                   if (bintime_cmp(&ffth->tick_time, &ffth->tick_time_lerp, >))
                           forward_jump = 1;
                   else
                           forward_jump = 0;
   
                   bintime_clear(&gap_lerp);
                   if (forward_jump) {
                           gap_lerp = ffth->tick_time;
                           bintime_sub(&gap_lerp, &ffth->tick_time_lerp);
                   } else {
                           gap_lerp = ffth->tick_time_lerp;
                           bintime_sub(&gap_lerp, &ffth->tick_time);
                   }
   
                   /*
                    * The reset from the RTC clock may be far from accurate, and
                    * reducing the gap between real time and interpolated time
                    * could take a very long time if the interpolated clock insists
                    * on strict monotonicity. The clock is reset under very strict
                    * conditions (kernel time is known to be wrong and
                    * synchronization daemon has been restarted recently.
                    * ffclock_boottime absorbs the jump to ensure boot time is
                    * correct and uptime functions stay consistent.
                    */
                   if (((ffclock_status & FFCLOCK_STA_UNSYNC) == FFCLOCK_STA_UNSYNC) &&
                       ((cest->status & FFCLOCK_STA_UNSYNC) == 0) &&
                       ((cest->status & FFCLOCK_STA_WARMUP) == FFCLOCK_STA_WARMUP)) {
                           if (forward_jump)
                                   bintime_add(&ffclock_boottime, &gap_lerp);
                           else
                                   bintime_sub(&ffclock_boottime, &gap_lerp);
                           ffth->tick_time_lerp = ffth->tick_time;
                           bintime_clear(&gap_lerp);
                   }
   
                   ffclock_status = cest->status;
                   ffth->period_lerp = cest->period;
   
                   /*
                    * Compute corrected period used for the linear interpolation of
                    * time. The rate of linear interpolation is capped to 5000PPM
                    * (5ms/s).
                    */
                   if (bintime_isset(&gap_lerp)) {
                           ffdelta = cest->update_ffcount;
                           ffdelta -= fftimehands->cest.update_ffcount;
                           ffclock_convert_delta(ffdelta, cest->period, &bt);
                           polling = bt.sec;
                           bt.sec = 0;
                           bt.frac = 5000000 * (uint64_t)18446744073LL;
                           bintime_mul(&bt, polling);
                           if (bintime_cmp(&gap_lerp, &bt, >))
                                   gap_lerp = bt;
   
                           /* Approximate 1 sec by 1-(1/2^64) to ease arithmetic */
                           frac = 0;
                           if (gap_lerp.sec > 0) {
                                   frac -= 1;
                                   frac /= ffdelta / gap_lerp.sec;
                           }
                           frac += gap_lerp.frac / ffdelta;
   
                           if (forward_jump)
                                   ffth->period_lerp += frac;
                           else
                                   ffth->period_lerp -= frac;
                   }
   
                   ffclock_updated = 0;
           }
           if (++ogen == 0)
                   ogen = 1;
           ffth->gen = ogen;
           fftimehands = ffth;
   }
   
   /*
    * Adjust the fftimehands when the timecounter is changed. Stating the obvious,
    * the old and new hardware counter cannot be read simultaneously. tc_windup()
    * does read the two counters 'back to back', but a few cycles are effectively
    * lost, and not accumulated in tick_ffcount. This is a fairly radical
    * operation for a feed-forward synchronization daemon, and it is its job to not
    * pushing irrelevant data to the kernel. Because there is no locking here,
    * simply force to ignore pending or next update to give daemon a chance to
    * realize the counter has changed.
    */
   static void
   ffclock_change_tc(struct timehands *th)
   {
           struct fftimehands *ffth;
           struct ffclock_estimate *cest;
           struct timecounter *tc;
           uint8_t ogen;
   
           tc = th->th_counter;
           ffth = fftimehands->next;
           ogen = ffth->gen;
           ffth->gen = 0;
   
           cest = &ffth->cest;
           bcopy(&(fftimehands->cest), cest, sizeof(struct ffclock_estimate));
           cest->period = ((1ULL << 63) / tc->tc_frequency ) << 1;
           cest->errb_abs = 0;
           cest->errb_rate = 0;
           cest->status |= FFCLOCK_STA_UNSYNC;
   
           ffth->tick_ffcount = fftimehands->tick_ffcount;
           ffth->tick_time_lerp = fftimehands->tick_time_lerp;
           ffth->tick_time = fftimehands->tick_time;
           ffth->period_lerp = cest->period;
   
           /* Do not lock but ignore next update from synchronization daemon. */
           ffclock_updated--;
   
           if (++ogen == 0)
                   ogen = 1;
           ffth->gen = ogen;
           fftimehands = ffth;
   }
   
   /*
    * Retrieve feed-forward counter and time of last kernel tick.
    */
   void
   ffclock_last_tick(ffcounter *ffcount, struct bintime *bt, uint32_t flags)
   {
           struct fftimehands *ffth;
           uint8_t gen;
   
           /*
            * No locking but check generation has not changed. Also need to make
            * sure ffdelta is positive, i.e. ffcount > tick_ffcount.
            */
           do {
                   ffth = fftimehands;
                   gen = ffth->gen;
                   if ((flags & FFCLOCK_LERP) == FFCLOCK_LERP)
                           *bt = ffth->tick_time_lerp;
                   else
                           *bt = ffth->tick_time;
                   *ffcount = ffth->tick_ffcount;
           } while (gen == 0 || gen != ffth->gen);
   }
   
   /*
    * Absolute clock conversion. Low level function to convert ffcounter to
    * bintime. The ffcounter is converted using the current ffclock period estimate
    * or the "interpolated period" to ensure monotonicity.
    * NOTE: this conversion may have been deferred, and the clock updated since the
    * hardware counter has been read.
    */
   void
   ffclock_convert_abs(ffcounter ffcount, struct bintime *bt, uint32_t flags)
   {
           struct fftimehands *ffth;
           struct bintime bt2;
           ffcounter ffdelta;
           uint8_t gen;
   
           /*
            * No locking but check generation has not changed. Also need to make
            * sure ffdelta is positive, i.e. ffcount > tick_ffcount.
            */
           do {
                   ffth = fftimehands;
                   gen = ffth->gen;
                   if (ffcount > ffth->tick_ffcount)
                           ffdelta = ffcount - ffth->tick_ffcount;
                   else
                           ffdelta = ffth->tick_ffcount - ffcount;
   
                   if ((flags & FFCLOCK_LERP) == FFCLOCK_LERP) {
                           *bt = ffth->tick_time_lerp;
                           ffclock_convert_delta(ffdelta, ffth->period_lerp, &bt2);
                   } else {
                           *bt = ffth->tick_time;
                           ffclock_convert_delta(ffdelta, ffth->cest.period, &bt2);
                   }
   
                   if (ffcount > ffth->tick_ffcount)
                           bintime_add(bt, &bt2);
                   else
                           bintime_sub(bt, &bt2);
           } while (gen == 0 || gen != ffth->gen);
   }
   
   /*
    * Difference clock conversion.
    * Low level function to Convert a time interval measured in RAW counter units
    * into bintime. The difference clock allows measuring small intervals much more
    * reliably than the absolute clock.
    */
   void
   ffclock_convert_diff(ffcounter ffdelta, struct bintime *bt)
   {
           struct fftimehands *ffth;
           uint8_t gen;
   
           /* No locking but check generation has not changed. */
           do {
                   ffth = fftimehands;
                   gen = ffth->gen;
                   ffclock_convert_delta(ffdelta, ffth->cest.period, bt);
           } while (gen == 0 || gen != ffth->gen);
   }
   
   /*
    * Access to current ffcounter value.
    */
   void
   ffclock_read_counter(ffcounter *ffcount)
   {
           struct timehands *th;
           struct fftimehands *ffth;
           unsigned int gen, delta;
   
           /*
            * ffclock_windup() called from tc_windup(), safe to rely on
            * th->th_generation only, for correct delta and ffcounter.
            */
           do {
                   th = timehands;
                   gen = atomic_load_acq_int(&th->th_generation);
                   ffth = fftimehands;
                   delta = tc_delta(th);
                   *ffcount = ffth->tick_ffcount;
                   atomic_thread_fence_acq();
           } while (gen == 0 || gen != th->th_generation);
   
           *ffcount += delta;
   }
   
   void
   binuptime(struct bintime *bt)
   {
   
           binuptime_fromclock(bt, sysclock_active);
   }
   
   void
   nanouptime(struct timespec *tsp)
   {
   
           nanouptime_fromclock(tsp, sysclock_active);
   }
   
   void
   microuptime(struct timeval *tvp)
   {
   
           microuptime_fromclock(tvp, sysclock_active);
   }
   
   void
   bintime(struct bintime *bt)
   {
   
           bintime_fromclock(bt, sysclock_active);
   }
   
   void
   nanotime(struct timespec *tsp)
   {
   
           nanotime_fromclock(tsp, sysclock_active);
   }
   
   void
   microtime(struct timeval *tvp)
   {
   
           microtime_fromclock(tvp, sysclock_active);
   }
   
   void
   getbinuptime(struct bintime *bt)
   {
   
           getbinuptime_fromclock(bt, sysclock_active);
   }
   
   void
   getnanouptime(struct timespec *tsp)
   {
   
           getnanouptime_fromclock(tsp, sysclock_active);
   }
   
   void
   getmicrouptime(struct timeval *tvp)
   {
   
           getmicrouptime_fromclock(tvp, sysclock_active);
   }
   
   void
   getbintime(struct bintime *bt)
   {
   
           getbintime_fromclock(bt, sysclock_active);
   }
   
   void
   getnanotime(struct timespec *tsp)
   {
   
           getnanotime_fromclock(tsp, sysclock_active);
   }
   
   void
   getmicrotime(struct timeval *tvp)
  {
  
          getmicrouptime_fromclock(tvp, sysclock_active);
  }
  
  #endif /* FFCLOCK */
  
  /*
   * This is a clone of getnanotime and used for walltimestamps.
   * The dtrace_ prefix prevents fbt from creating probes for
   * it so walltimestamp can be safely used in all fbt probes.
   */
  void
  dtrace_getnanotime(struct timespec *tsp)
  {
  
          GETTHMEMBER(tsp, th_nanotime);
  }
  
  /*
   * This is a clone of getnanouptime used for time since boot.
   * The dtrace_ prefix prevents fbt from creating probes for
   * it so an uptime that can be safely used in all fbt probes.
   */
  void
  dtrace_getnanouptime(struct timespec *tsp)
  {
          struct bintime bt;
  
          GETTHMEMBER(&bt, th_offset);
          bintime2timespec(&bt, tsp);
  }
  
  /*
   * System clock currently providing time to the system. Modifiable via sysctl
   * when the FFCLOCK option is defined.
   */
  int sysclock_active = SYSCLOCK_FBCK;
  
  /* Internal NTP status and error estimates. */
  extern int time_status;
  extern long time_esterror;
  
  /*
   * Take a snapshot of sysclock data which can be used to compare system clocks
   * and generate timestamps after the fact.
   */
  void
  sysclock_getsnapshot(struct sysclock_snap *clock_snap, int fast)
  {
          struct fbclock_info *fbi;
          struct timehands *th;
          struct bintime bt;
          unsigned int delta, gen;
  #ifdef FFCLOCK
          ffcounter ffcount;
          struct fftimehands *ffth;
          struct ffclock_info *ffi;
          struct ffclock_estimate cest;
  
          ffi = &clock_snap->ff_info;
  #endif
  
          fbi = &clock_snap->fb_info;
          delta = 0;
  
          do {
                  th = timehands;
                  gen = atomic_load_acq_int(&th->th_generation);
                  fbi->th_scale = th->th_scale;
                  fbi->tick_time = th->th_offset;
  #ifdef FFCLOCK
                  ffth = fftimehands;
                  ffi->tick_time = ffth->tick_time_lerp;
                  ffi->tick_time_lerp = ffth->tick_time_lerp;
                  ffi->period = ffth->cest.period;
                  ffi->period_lerp = ffth->period_lerp;
                  clock_snap->ffcount = ffth->tick_ffcount;
                  cest = ffth->cest;
  #endif
                  if (!fast)
                          delta = tc_delta(th);
                  atomic_thread_fence_acq();
          } while (gen == 0 || gen != th->th_generation);
  
          clock_snap->delta = delta;
          clock_snap->sysclock_active = sysclock_active;
  
          /* Record feedback clock status and error. */
          clock_snap->fb_info.status = time_status;
          /* XXX: Very crude estimate of feedback clock error. */
          bt.sec = time_esterror / 1000000;
          bt.frac = ((time_esterror - bt.sec) * 1000000) *
              (uint64_t)18446744073709ULL;
          clock_snap->fb_info.error = bt;
  
  #ifdef FFCLOCK
          if (!fast)
                  clock_snap->ffcount += delta;
  
          /* Record feed-forward clock leap second adjustment. */
          ffi->leapsec_adjustment = cest.leapsec_total;
          if (clock_snap->ffcount > cest.leapsec_next)
                  ffi->leapsec_adjustment -= cest.leapsec;
  
          /* Record feed-forward clock status and error. */
          clock_snap->ff_info.status = cest.status;
          ffcount = clock_snap->ffcount - cest.update_ffcount;
          ffclock_convert_delta(ffcount, cest.period, &bt);
          /* 18446744073709 = int(2^64/1e12), err_bound_rate in [ps/s]. */
          bintime_mul(&bt, cest.errb_rate * (uint64_t)18446744073709ULL);
          /* 18446744073 = int(2^64 / 1e9), since err_abs in [ns]. */
          bintime_addx(&bt, cest.errb_abs * (uint64_t)18446744073ULL);
          clock_snap->ff_info.error = bt;
  #endif
  }
  
  /*
   * Convert a sysclock snapshot into a struct bintime based on the specified
   * clock source and flags.
   */
  int
  sysclock_snap2bintime(struct sysclock_snap *cs, struct bintime *bt,
      int whichclock, uint32_t flags)
  {
          struct bintime boottimebin;
  #ifdef FFCLOCK
          struct bintime bt2;
          uint64_t period;
  #endif
  
          switch (whichclock) {
          case SYSCLOCK_FBCK:
                  *bt = cs->fb_info.tick_time;
  
                  /* If snapshot was created with !fast, delta will be >0. */
                  if (cs->delta > 0)
                          bintime_addx(bt, cs->fb_info.th_scale * cs->delta);
  
                  if ((flags & FBCLOCK_UPTIME) == 0) {
                          getboottimebin(&boottimebin);
                          bintime_add(bt, &boottimebin);
                  }
                  break;
  #ifdef FFCLOCK
          case SYSCLOCK_FFWD:
                  if (flags & FFCLOCK_LERP) {
                          *bt = cs->ff_info.tick_time_lerp;
                          period = cs->ff_info.period_lerp;
                  } else {
                          *bt = cs->ff_info.tick_time;
                          period = cs->ff_info.period;
                  }
  
                  /* If snapshot was created with !fast, delta will be >0. */
                  if (cs->delta > 0) {
                          ffclock_convert_delta(cs->delta, period, &bt2);
                          bintime_add(bt, &bt2);
                  }
  
                  /* Leap second adjustment. */
                  if (flags & FFCLOCK_LEAPSEC)
                          bt->sec -= cs->ff_info.leapsec_adjustment;
  
                  /* Boot time adjustment, for uptime/monotonic clocks. */
                  if (flags & FFCLOCK_UPTIME)
                          bintime_sub(bt, &ffclock_boottime);
                  break;
  #endif
          default:
                  return (EINVAL);
                  break;
          }
  
          return (0);
  }
  
  /*
   * Initialize a new timecounter and possibly use it.
   */
  void
  tc_init(struct timecounter *tc)
  {
          u_int u;
          struct sysctl_oid *tc_root;
  
          u = tc->tc_frequency / tc->tc_counter_mask;
          /* XXX: We need some margin here, 10% is a guess */
          u *= 11;
          u /= 10;
          if (u > hz && tc->tc_quality >= 0) {
                  tc->tc_quality = -2000;
                  if (bootverbose) {
                          printf("Timecounter \"%s\" frequency %ju Hz",
                              tc->tc_name, (uintmax_t)tc->tc_frequency);
                          printf(" -- Insufficient hz, needs at least %u\n", u);
                  }
          } else if (tc->tc_quality >= 0 || bootverbose) {
                  printf("Timecounter \"%s\" frequency %ju Hz quality %d\n",
                      tc->tc_name, (uintmax_t)tc->tc_frequency,
                      tc->tc_quality);
          }
  
          /*
           * Set up sysctl tree for this counter.
           */
          tc_root = SYSCTL_ADD_NODE_WITH_LABEL(NULL,
              SYSCTL_STATIC_CHILDREN(_kern_timecounter_tc), OID_AUTO, tc->tc_name,
              CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
              "timecounter description", "timecounter");
          SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(tc_root), OID_AUTO,
              "mask", CTLFLAG_RD, &(tc->tc_counter_mask), 0,
              "mask for implemented bits");
          SYSCTL_ADD_PROC(NULL, SYSCTL_CHILDREN(tc_root), OID_AUTO,
              "counter", CTLTYPE_UINT | CTLFLAG_RD | CTLFLAG_MPSAFE, tc,
              sizeof(*tc), sysctl_kern_timecounter_get, "IU",
              "current timecounter value");
          SYSCTL_ADD_PROC(NULL, SYSCTL_CHILDREN(tc_root), OID_AUTO,
              "frequency", CTLTYPE_U64 | CTLFLAG_RD | CTLFLAG_MPSAFE, tc,
              sizeof(*tc), sysctl_kern_timecounter_freq, "QU",
              "timecounter frequency");
          SYSCTL_ADD_INT(NULL, SYSCTL_CHILDREN(tc_root), OID_AUTO,
              "quality", CTLFLAG_RD, &(tc->tc_quality), 0,
              "goodness of time counter");
  
          mtx_lock(&tc_lock);
          tc->tc_next = timecounters;
          timecounters = tc;
  
          /*
           * Do not automatically switch if the current tc was specifically
           * chosen.  Never automatically use a timecounter with negative quality.
           * Even though we run on the dummy counter, switching here may be
           * worse since this timecounter may not be monotonic.
           */
          if (tc_chosen)
                  goto unlock;
          if (tc->tc_quality < 0)
                  goto unlock;
          if (tc_from_tunable[0] != '\0' &&
              strcmp(tc->tc_name, tc_from_tunable) == 0) {
                  tc_chosen = 1;
                  tc_from_tunable[0] = '\0';
          } else {
                  if (tc->tc_quality < timecounter->tc_quality)
                          goto unlock;
                  if (tc->tc_quality == timecounter->tc_quality &&
                      tc->tc_frequency < timecounter->tc_frequency)
                          goto unlock;
          }
          (void)tc->tc_get_timecount(tc);
          timecounter = tc;
  unlock:
          mtx_unlock(&tc_lock);
  }
  
  /* Report the frequency of the current timecounter. */
  uint64_t
  tc_getfrequency(void)
  {
  
          return (timehands->th_counter->tc_frequency);
  }
  
  static bool
  sleeping_on_old_rtc(struct thread *td)
  {
  
          /*
           * td_rtcgen is modified by curthread when it is running,
           * and by other threads in this function.  By finding the thread
           * on a sleepqueue and holding the lock on the sleepqueue
           * chain, we guarantee that the thread is not running and that
           * modifying td_rtcgen is safe.  Setting td_rtcgen to zero informs
           * the thread that it was woken due to a real-time clock adjustment.
           * (The declaration of td_rtcgen refers to this comment.)
           */
          if (td->td_rtcgen != 0 && td->td_rtcgen != rtc_generation) {
                  td->td_rtcgen = 0;
                  return (true);
          }
          return (false);
  }
  
  static struct mtx tc_setclock_mtx;
  MTX_SYSINIT(tc_setclock_init, &tc_setclock_mtx, "tcsetc", MTX_SPIN);
  
  /*
   * Step our concept of UTC.  This is done by modifying our estimate of
   * when we booted.
   */
  void
  tc_setclock(struct timespec *ts)
  {
          struct timespec tbef, taft;
          struct bintime bt, bt2;
  
          timespec2bintime(ts, &bt);
          nanotime(&tbef);
          mtx_lock_spin(&tc_setclock_mtx);
          cpu_tick_calibrate(1);
          binuptime(&bt2);
          bintime_sub(&bt, &bt2);
  
          /* XXX fiddle all the little crinkly bits around the fiords... */
          tc_windup(&bt);
          mtx_unlock_spin(&tc_setclock_mtx);
  
          /* Avoid rtc_generation == 0, since td_rtcgen == 0 is special. */
          atomic_add_rel_int(&rtc_generation, 2);
          sleepq_chains_remove_matching(sleeping_on_old_rtc);
          if (timestepwarnings) {
                  nanotime(&taft);
                  log(LOG_INFO,
                      "Time stepped from %jd.%09ld to %jd.%09ld (%jd.%09ld)\n",
                      (intmax_t)tbef.tv_sec, tbef.tv_nsec,
                      (intmax_t)taft.tv_sec, taft.tv_nsec,
                      (intmax_t)ts->tv_sec, ts->tv_nsec);
          }
  }
  
  /*
   * Recalculate the scaling factor.  We want the number of 1/2^64
   * fractions of a second per period of the hardware counter, taking
   * into account the th_adjustment factor which the NTP PLL/adjtime(2)
   * processing provides us with.
   *
   * The th_adjustment is nanoseconds per second with 32 bit binary
   * fraction and we want 64 bit binary fraction of second:
   *
   *       x = a * 2^32 / 10^9 = a * 4.294967296
   *
   * The range of th_adjustment is +/- 5000PPM so inside a 64bit int
   * we can only multiply by about 850 without overflowing, that
   * leaves no suitably precise fractions for multiply before divide.
   *
   * Divide before multiply with a fraction of 2199/512 results in a
   * systematic undercompensation of 10PPM of th_adjustment.  On a
   * 5000PPM adjustment this is a 0.05PPM error.  This is acceptable.
   *
   * We happily sacrifice the lowest of the 64 bits of our result
   * to the goddess of code clarity.
   */
  static void
  recalculate_scaling_factor_and_large_delta(struct timehands *th)
  {
          uint64_t scale;
  
          scale = (uint64_t)1 << 63;
          scale += (th->th_adjustment / 1024) * 2199;
          scale /= th->th_counter->tc_frequency;
          th->th_scale = scale * 2;
          th->th_large_delta = MIN(((uint64_t)1 << 63) / scale, UINT_MAX);
  }
  
  /*
   * Initialize the next struct timehands in the ring and make
   * it the active timehands.  Along the way we might switch to a different
   * timecounter and/or do seconds processing in NTP.  Slightly magic.
   */
  static void
  tc_windup(struct bintime *new_boottimebin)
  {
          struct bintime bt;
          struct timecounter *tc;
          struct timehands *th, *tho;
          u_int delta, ncount, ogen;
          int i;
          time_t t;
  
          /*
           * Make the next timehands a copy of the current one, but do
           * not overwrite the generation or next pointer.  While we
           * update the contents, the generation must be zero.  We need
           * to ensure that the zero generation is visible before the
           * data updates become visible, which requires release fence.
           * For similar reasons, re-reading of the generation after the
           * data is read should use acquire fence.
           */
          tho = timehands;
          th = tho->th_next;
          ogen = th->th_generation;
          th->th_generation = 0;
          atomic_thread_fence_rel();
          memcpy(th, tho, offsetof(struct timehands, th_generation));
          if (new_boottimebin != NULL)
                  th->th_boottime = *new_boottimebin;
  
          /*
           * Capture a timecounter delta on the current timecounter and if
           * changing timecounters, a counter value from the new timecounter.
           * Update the offset fields accordingly.
           */
          tc = atomic_load_ptr(&timecounter);
          delta = tc_delta(th);
          if (th->th_counter != tc)
                  ncount = tc->tc_get_timecount(tc);
          else
                  ncount = 0;
  #ifdef FFCLOCK
          ffclock_windup(delta);
  #endif
          th->th_offset_count += delta;
          th->th_offset_count &= th->th_counter->tc_counter_mask;
          bintime_add_tc_delta(&th->th_offset, th->th_scale,
              th->th_large_delta, delta);
  
          /*
           * Hardware latching timecounters may not generate interrupts on
           * PPS events, so instead we poll them.  There is a finite risk that
           * the hardware might capture a count which is later than the one we
           * got above, and therefore possibly in the next NTP second which might
           * have a different rate than the current NTP second.  It doesn't
           * matter in practice.
           */
          if (tho->th_counter->tc_poll_pps)
                  tho->th_counter->tc_poll_pps(tho->th_counter);
  
          /*
           * Deal with NTP second processing.  The loop normally
           * iterates at most once, but in extreme situations it might
           * keep NTP sane if timeouts are not run for several seconds.
           * At boot, the time step can be large when the TOD hardware
           * has been read, so on really large steps, we call
           * ntp_update_second only twice.  We need to call it twice in
           * case we missed a leap second.
           */
          bt = th->th_offset;
          bintime_add(&bt, &th->th_boottime);
          i = bt.sec - tho->th_microtime.tv_sec;
          if (i > 0) {
                  if (i > LARGE_STEP)
                          i = 2;
  
                  do {
                          t = bt.sec;
                          ntp_update_second(&th->th_adjustment, &bt.sec);
                          if (bt.sec != t)
                                  th->th_boottime.sec += bt.sec - t;
                          --i;
                  } while (i > 0);
  
                  recalculate_scaling_factor_and_large_delta(th);
          }
  
          /* Update the UTC timestamps used by the get*() functions. */
          th->th_bintime = bt;
          bintime2timeval(&bt, &th->th_microtime);
          bintime2timespec(&bt, &th->th_nanotime);
  
          /* Now is a good time to change timecounters. */
          if (th->th_counter != tc) {
  #ifndef __arm__
                  if ((tc->tc_flags & TC_FLAGS_C2STOP) != 0)
                          cpu_disable_c2_sleep++;
                  if ((th->th_counter->tc_flags & TC_FLAGS_C2STOP) != 0)
                          cpu_disable_c2_sleep--;
  #endif
                  th->th_counter = tc;
                  th->th_offset_count = ncount;
                  tc_min_ticktock_freq = max(1, tc->tc_frequency /
                      (((uint64_t)tc->tc_counter_mask + 1) / 3));
                  recalculate_scaling_factor_and_large_delta(th);
  #ifdef FFCLOCK
                  ffclock_change_tc(th);
  #endif
          }
  
          /*
           * Now that the struct timehands is again consistent, set the new
           * generation number, making sure to not make it zero.
           */
          if (++ogen == 0)
                  ogen = 1;
          atomic_store_rel_int(&th->th_generation, ogen);
  
          /* Go live with the new struct timehands. */
  #ifdef FFCLOCK
          switch (sysclock_active) {
          case SYSCLOCK_FBCK:
  #endif
                  time_second = th->th_microtime.tv_sec;
                  time_uptime = th->th_offset.sec;
  #ifdef FFCLOCK
                  break;
          case SYSCLOCK_FFWD:
                  time_second = fftimehands->tick_time_lerp.sec;
                  time_uptime = fftimehands->tick_time_lerp.sec - ffclock_boottime.sec;
                  break;
          }
  #endif
  
          timehands = th;
          timekeep_push_vdso();
  }
  
  /* Report or change the active timecounter hardware. */
  static int
  sysctl_kern_timecounter_hardware(SYSCTL_HANDLER_ARGS)
  {
          char newname[32];
          struct timecounter *newtc, *tc;
          int error;
  
          mtx_lock(&tc_lock);
          tc = timecounter;
          strlcpy(newname, tc->tc_name, sizeof(newname));
          mtx_unlock(&tc_lock);
  
          error = sysctl_handle_string(oidp, &newname[0], sizeof(newname), req);
          if (error != 0 || req->newptr == NULL)
                  return (error);
  
          mtx_lock(&tc_lock);
          /* Record that the tc in use now was specifically chosen. */
          tc_chosen = 1;
          if (strcmp(newname, tc->tc_name) == 0) {
                  mtx_unlock(&tc_lock);
                  return (0);
          }
          for (newtc = timecounters; newtc != NULL; newtc = newtc->tc_next) {
                  if (strcmp(newname, newtc->tc_name) != 0)
                          continue;
  
                  /* Warm up new timecounter. */
                  (void)newtc->tc_get_timecount(newtc);
  
                  timecounter = newtc;
  
                  /*
                   * The vdso timehands update is deferred until the next
                   * 'tc_windup()'.
                   *
                   * This is prudent given that 'timekeep_push_vdso()' does not
                   * use any locking and that it can be called in hard interrupt
                   * context via 'tc_windup()'.
                   */
                  break;
          }
          mtx_unlock(&tc_lock);
          return (newtc != NULL ? 0 : EINVAL);
  }
  SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware,
      CTLTYPE_STRING | CTLFLAG_RWTUN | CTLFLAG_NOFETCH | CTLFLAG_MPSAFE, 0, 0,
      sysctl_kern_timecounter_hardware, "A",
      "Timecounter hardware selected");
  
  /* Report the available timecounter hardware. */
  static int
  sysctl_kern_timecounter_choice(SYSCTL_HANDLER_ARGS)
  {
          struct sbuf sb;
          struct timecounter *tc;
          int error;
  
          error = sysctl_wire_old_buffer(req, 0);
          if (error != 0)
                  return (error);
          sbuf_new_for_sysctl(&sb, NULL, 0, req);
          mtx_lock(&tc_lock);
          for (tc = timecounters; tc != NULL; tc = tc->tc_next) {
                  if (tc != timecounters)
                          sbuf_putc(&sb, ' ');
                  sbuf_printf(&sb, "%s(%d)", tc->tc_name, tc->tc_quality);
          }
          mtx_unlock(&tc_lock);
          error = sbuf_finish(&sb);
          sbuf_delete(&sb);
          return (error);
  }
  
  SYSCTL_PROC(_kern_timecounter, OID_AUTO, choice,
      CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE, 0, 0,
      sysctl_kern_timecounter_choice, "A",
      "Timecounter hardware detected");
  
  /*
   * RFC 2783 PPS-API implementation.
   */
  
  /*
   *  Return true if the driver is aware of the abi version extensions in the
   *  pps_state structure, and it supports at least the given abi version number.
   */
  static inline int
  abi_aware(struct pps_state *pps, int vers)
  {
  
          return ((pps->kcmode & KCMODE_ABIFLAG) && pps->driver_abi >= vers);
  }
  
  static int
  pps_fetch(struct pps_fetch_args *fapi, struct pps_state *pps)
  {
          int err, timo;
          pps_seq_t aseq, cseq;
          struct timeval tv;
  
          if (fapi->tsformat && fapi->tsformat != PPS_TSFMT_TSPEC)
                  return (EINVAL);
  
          /*
           * If no timeout is requested, immediately return whatever values were
           * most recently captured.  If timeout seconds is -1, that's a request
           * to block without a timeout.  WITNESS won't let us sleep forever
           * without a lock (we really don't need a lock), so just repeatedly
           * sleep a long time.
           */
          if (fapi->timeout.tv_sec || fapi->timeout.tv_nsec) {
                  if (fapi->timeout.tv_sec == -1)
                          timo = 0x7fffffff;
                  else {
                          tv.tv_sec = fapi->timeout.tv_sec;
                          tv.tv_usec = fapi->timeout.tv_nsec / 1000;
                          timo = tvtohz(&tv);
                  }
                  aseq = atomic_load_int(&pps->ppsinfo.assert_sequence);
                  cseq = atomic_load_int(&pps->ppsinfo.clear_sequence);
                  while (aseq == atomic_load_int(&pps->ppsinfo.assert_sequence) &&
                      cseq == atomic_load_int(&pps->ppsinfo.clear_sequence)) {
                          if (abi_aware(pps, 1) && pps->driver_mtx != NULL) {
                                  if (pps->flags & PPSFLAG_MTX_SPIN) {
                                          err = msleep_spin(pps, pps->driver_mtx,
                                              "ppsfch", timo);
                                  } else {
                                          err = msleep(pps, pps->driver_mtx, PCATCH,
                                              "ppsfch", timo);
                                  }
                          } else {
                                  err = tsleep(pps, PCATCH, "ppsfch", timo);
                          }
                          if (err == EWOULDBLOCK) {
                                  if (fapi->timeout.tv_sec == -1) {
                                          continue;
                                  } else {
                                          return (ETIMEDOUT);
                                  }
                          } else if (err != 0) {
                                  return (err);
                          }
                  }
          }
  
          pps->ppsinfo.current_mode = pps->ppsparam.mode;
          fapi->pps_info_buf = pps->ppsinfo;
  
          return (0);
  }
  
  int
  pps_ioctl(u_long cmd, caddr_t data, struct pps_state *pps)
  {
          pps_params_t *app;
          struct pps_fetch_args *fapi;
  #ifdef FFCLOCK
          struct pps_fetch_ffc_args *fapi_ffc;
  #endif
  #ifdef PPS_SYNC
          struct pps_kcbind_args *kapi;
  #endif
  
          KASSERT(pps != NULL, ("NULL pps pointer in pps_ioctl"));
          switch (cmd) {
          case PPS_IOC_CREATE:
                  return (0);
          case PPS_IOC_DESTROY:
                  return (0);
          case PPS_IOC_SETPARAMS:
                  app = (pps_params_t *)data;
                  if (app->mode & ~pps->ppscap)
                          return (EINVAL);
  #ifdef FFCLOCK
                  /* Ensure only a single clock is selected for ffc timestamp. */
                  if ((app->mode & PPS_TSCLK_MASK) == PPS_TSCLK_MASK)
                          return (EINVAL);
  #endif
                  pps->ppsparam = *app;
                  return (0);
          case PPS_IOC_GETPARAMS:
                  app = (pps_params_t *)data;
                  *app = pps->ppsparam;
                  app->api_version = PPS_API_VERS_1;
                  return (0);
          case PPS_IOC_GETCAP:
                  *(int*)data = pps->ppscap;
                  return (0);
          case PPS_IOC_FETCH:
                  fapi = (struct pps_fetch_args *)data;
                  return (pps_fetch(fapi, pps));
  #ifdef FFCLOCK
          case PPS_IOC_FETCH_FFCOUNTER:
                  fapi_ffc = (struct pps_fetch_ffc_args *)data;
                  if (fapi_ffc->tsformat && fapi_ffc->tsformat !=
                      PPS_TSFMT_TSPEC)
                          return (EINVAL);
                  if (fapi_ffc->timeout.tv_sec || fapi_ffc->timeout.tv_nsec)
                          return (EOPNOTSUPP);
                  pps->ppsinfo_ffc.current_mode = pps->ppsparam.mode;
                  fapi_ffc->pps_info_buf_ffc = pps->ppsinfo_ffc;
                  /* Overwrite timestamps if feedback clock selected. */
                  switch (pps->ppsparam.mode & PPS_TSCLK_MASK) {
                  case PPS_TSCLK_FBCK:
                          fapi_ffc->pps_info_buf_ffc.assert_timestamp =
                              pps->ppsinfo.assert_timestamp;
                          fapi_ffc->pps_info_buf_ffc.clear_timestamp =
                              pps->ppsinfo.clear_timestamp;
                          break;
                  case PPS_TSCLK_FFWD:
                          break;
                  default:
                          break;
                  }
                  return (0);
  #endif /* FFCLOCK */
          case PPS_IOC_KCBIND:
  #ifdef PPS_SYNC
                  kapi = (struct pps_kcbind_args *)data;
                  /* XXX Only root should be able to do this */
                  if (kapi->tsformat && kapi->tsformat != PPS_TSFMT_TSPEC)
                          return (EINVAL);
                  if (kapi->kernel_consumer != PPS_KC_HARDPPS)
                          return (EINVAL);
                  if (kapi->edge & ~pps->ppscap)
                          return (EINVAL);
                  pps->kcmode = (kapi->edge & KCMODE_EDGEMASK) |
                      (pps->kcmode & KCMODE_ABIFLAG);
                  return (0);
  #else
                  return (EOPNOTSUPP);
  #endif
          default:
                  return (ENOIOCTL);
          }
  }
  
  void
  pps_init(struct pps_state *pps)
  {
          pps->ppscap |= PPS_TSFMT_TSPEC | PPS_CANWAIT;
          if (pps->ppscap & PPS_CAPTUREASSERT)
                  pps->ppscap |= PPS_OFFSETASSERT;
          if (pps->ppscap & PPS_CAPTURECLEAR)
                  pps->ppscap |= PPS_OFFSETCLEAR;
  #ifdef FFCLOCK
          pps->ppscap |= PPS_TSCLK_MASK;
  #endif
          pps->kcmode &= ~KCMODE_ABIFLAG;
  }
  
  void
  pps_init_abi(struct pps_state *pps)
  {
  
          pps_init(pps);
          if (pps->driver_abi > 0) {
                  pps->kcmode |= KCMODE_ABIFLAG;
                  pps->kernel_abi = PPS_ABI_VERSION;
          }
  }
  
  void
  pps_capture(struct pps_state *pps)
  {
          struct timehands *th;
  
          KASSERT(pps != NULL, ("NULL pps pointer in pps_capture"));
          th = timehands;
          pps->capgen = atomic_load_acq_int(&th->th_generation);
          pps->capth = th;
  #ifdef FFCLOCK
          pps->capffth = fftimehands;
  #endif
          pps->capcount = th->th_counter->tc_get_timecount(th->th_counter);
          atomic_thread_fence_acq();
          if (pps->capgen != th->th_generation)
                  pps->capgen = 0;
  }
  
  void
  pps_event(struct pps_state *pps, int event)
  {
          struct bintime bt;
          struct timespec ts, *tsp, *osp;
          u_int tcount, *pcount;
          int foff;
          pps_seq_t *pseq;
  #ifdef FFCLOCK
          struct timespec *tsp_ffc;
          pps_seq_t *pseq_ffc;
          ffcounter *ffcount;
  #endif
  #ifdef PPS_SYNC
          int fhard;
  #endif
  
          KASSERT(pps != NULL, ("NULL pps pointer in pps_event"));
          /* Nothing to do if not currently set to capture this event type. */
          if ((event & pps->ppsparam.mode) == 0)
                  return;
          /* If the timecounter was wound up underneath us, bail out. */
          if (pps->capgen == 0 || pps->capgen !=
              atomic_load_acq_int(&pps->capth->th_generation))
                  return;
  
          /* Things would be easier with arrays. */
          if (event == PPS_CAPTUREASSERT) {
                  tsp = &pps->ppsinfo.assert_timestamp;
                  osp = &pps->ppsparam.assert_offset;
                  foff = pps->ppsparam.mode & PPS_OFFSETASSERT;
  #ifdef PPS_SYNC
                  fhard = pps->kcmode & PPS_CAPTUREASSERT;
  #endif
                  pcount = &pps->ppscount[0];
                  pseq = &pps->ppsinfo.assert_sequence;
  #ifdef FFCLOCK
                  ffcount = &pps->ppsinfo_ffc.assert_ffcount;
                  tsp_ffc = &pps->ppsinfo_ffc.assert_timestamp;
                  pseq_ffc = &pps->ppsinfo_ffc.assert_sequence;
  #endif
          } else {
                  tsp = &pps->ppsinfo.clear_timestamp;
                  osp = &pps->ppsparam.clear_offset;
                  foff = pps->ppsparam.mode & PPS_OFFSETCLEAR;
  #ifdef PPS_SYNC
                  fhard = pps->kcmode & PPS_CAPTURECLEAR;
  #endif
                  pcount = &pps->ppscount[1];
                  pseq = &pps->ppsinfo.clear_sequence;
  #ifdef FFCLOCK
                  ffcount = &pps->ppsinfo_ffc.clear_ffcount;
                  tsp_ffc = &pps->ppsinfo_ffc.clear_timestamp;
                  pseq_ffc = &pps->ppsinfo_ffc.clear_sequence;
  #endif
          }
  
          /*
           * If the timecounter changed, we cannot compare the count values, so
           * we have to drop the rest of the PPS-stuff until the next event.
           */
          if (pps->ppstc != pps->capth->th_counter) {
                  pps->ppstc = pps->capth->th_counter;
                  *pcount = pps->capcount;
                  pps->ppscount[2] = pps->capcount;
                  return;
          }
  
          /* Convert the count to a timespec. */
          tcount = pps->capcount - pps->capth->th_offset_count;
          tcount &= pps->capth->th_counter->tc_counter_mask;
          bt = pps->capth->th_bintime;
          bintime_addx(&bt, pps->capth->th_scale * tcount);
          bintime2timespec(&bt, &ts);
  
          /* If the timecounter was wound up underneath us, bail out. */
          atomic_thread_fence_acq();
          if (pps->capgen != pps->capth->th_generation)
                  return;
  
          *pcount = pps->capcount;
          (*pseq)++;
          *tsp = ts;
  
          if (foff) {
                  timespecadd(tsp, osp, tsp);
                  if (tsp->tv_nsec < 0) {
                          tsp->tv_nsec += 1000000000;
                          tsp->tv_sec -= 1;
                  }
          }
  
  #ifdef FFCLOCK
          *ffcount = pps->capffth->tick_ffcount + tcount;
          bt = pps->capffth->tick_time;
          ffclock_convert_delta(tcount, pps->capffth->cest.period, &bt);
          bintime_add(&bt, &pps->capffth->tick_time);
          bintime2timespec(&bt, &ts);
          (*pseq_ffc)++;
          *tsp_ffc = ts;
  #endif
  
  #ifdef PPS_SYNC
          if (fhard) {
                  uint64_t scale;
  
                  /*
                   * Feed the NTP PLL/FLL.
                   * The FLL wants to know how many (hardware) nanoseconds
                   * elapsed since the previous event.
                   */
                  tcount = pps->capcount - pps->ppscount[2];
                  pps->ppscount[2] = pps->capcount;
                  tcount &= pps->capth->th_counter->tc_counter_mask;
                  scale = (uint64_t)1 << 63;
                  scale /= pps->capth->th_counter->tc_frequency;
                  scale *= 2;
                  bt.sec = 0;
                  bt.frac = 0;
                  bintime_addx(&bt, scale * tcount);
                  bintime2timespec(&bt, &ts);
                  hardpps(tsp, ts.tv_nsec + 1000000000 * ts.tv_sec);
          }
  #endif
  
          /* Wakeup anyone sleeping in pps_fetch().  */
          wakeup(pps);
  }
  
  /*
   * Timecounters need to be updated every so often to prevent the hardware
   * counter from overflowing.  Updating also recalculates the cached values
   * used by the get*() family of functions, so their precision depends on
   * the update frequency.
   */
  
  static int tc_tick;
  SYSCTL_INT(_kern_timecounter, OID_AUTO, tick, CTLFLAG_RD, &tc_tick, 0,
      "Approximate number of hardclock ticks in a millisecond");
  
  void
  tc_ticktock(int cnt)
  {
          static int count;
  
          if (mtx_trylock_spin(&tc_setclock_mtx)) {
                  count += cnt;
                  if (count >= tc_tick) {
                          count = 0;
                          tc_windup(NULL);
                  }
                  mtx_unlock_spin(&tc_setclock_mtx);
          }
  }
  
  static void __inline
  tc_adjprecision(void)
  {
          int t;
  
          if (tc_timepercentage > 0) {
                  t = (99 + tc_timepercentage) / tc_timepercentage;
                  tc_precexp = fls(t + (t >> 1)) - 1;
                  FREQ2BT(hz / tc_tick, &bt_timethreshold);
                  FREQ2BT(hz, &bt_tickthreshold);
                  bintime_shift(&bt_timethreshold, tc_precexp);
                  bintime_shift(&bt_tickthreshold, tc_precexp);
          } else {
                  tc_precexp = 31;
                  bt_timethreshold.sec = INT_MAX;
                  bt_timethreshold.frac = ~(uint64_t)0;
                  bt_tickthreshold = bt_timethreshold;
          }
          sbt_timethreshold = bttosbt(bt_timethreshold);
          sbt_tickthreshold = bttosbt(bt_tickthreshold);
  }
  
  static int
  sysctl_kern_timecounter_adjprecision(SYSCTL_HANDLER_ARGS)
  {
          int error, val;
  
          val = tc_timepercentage;
          error = sysctl_handle_int(oidp, &val, 0, req);
          if (error != 0 || req->newptr == NULL)
                  return (error);
          tc_timepercentage = val;
          if (cold)
                  goto done;
          tc_adjprecision();
  done:
          return (0);
  }
  
  /* Set up the requested number of timehands. */
  static void
  inittimehands(void *dummy)
  {
          struct timehands *thp;
          int i;
  
          TUNABLE_INT_FETCH("kern.timecounter.timehands_count",
              &timehands_count);
          if (timehands_count < 1)
                  timehands_count = 1;
          if (timehands_count > nitems(ths))
                  timehands_count = nitems(ths);
          for (i = 1, thp = &ths[0]; i < timehands_count;  thp = &ths[i++])
                  thp->th_next = &ths[i];
          thp->th_next = &ths[0];
  
          TUNABLE_STR_FETCH("kern.timecounter.hardware", tc_from_tunable,
              sizeof(tc_from_tunable));
  
          mtx_init(&tc_lock, "tc", NULL, MTX_DEF);
  }
  SYSINIT(timehands, SI_SUB_TUNABLES, SI_ORDER_ANY, inittimehands, NULL);
  
  static void
  inittimecounter(void *dummy)
  {
          u_int p;
          int tick_rate;
  
          /*
           * Set the initial timeout to
           * max(1, <approx. number of hardclock ticks in a millisecond>).
           * People should probably not use the sysctl to set the timeout
           * to smaller than its initial value, since that value is the
           * smallest reasonable one.  If they want better timestamps they
           * should use the non-"get"* functions.
           */
          if (hz > 1000)
                  tc_tick = (hz + 500) / 1000;
          else
                  tc_tick = 1;
          tc_adjprecision();
          FREQ2BT(hz, &tick_bt);
          tick_sbt = bttosbt(tick_bt);
          tick_rate = hz / tc_tick;
          FREQ2BT(tick_rate, &tc_tick_bt);
          tc_tick_sbt = bttosbt(tc_tick_bt);
          p = (tc_tick * 1000000) / hz;
          printf("Timecounters tick every %d.%03u msec\n", p / 1000, p % 1000);
  
  #ifdef FFCLOCK
          ffclock_init();
  #endif
  
          /* warm up new timecounter (again) and get rolling. */
          (void)timecounter->tc_get_timecount(timecounter);
          mtx_lock_spin(&tc_setclock_mtx);
          tc_windup(NULL);
          mtx_unlock_spin(&tc_setclock_mtx);
  }
  
  SYSINIT(timecounter, SI_SUB_CLOCKS, SI_ORDER_SECOND, inittimecounter, NULL);
  
  /* Cpu tick handling -------------------------------------------------*/
  
  static bool cpu_tick_variable;
  static uint64_t cpu_tick_frequency;
  
  DPCPU_DEFINE_STATIC(uint64_t, tc_cpu_ticks_base);
  DPCPU_DEFINE_STATIC(unsigned, tc_cpu_ticks_last);
  
  static uint64_t
  tc_cpu_ticks(void)
  {
          struct timecounter *tc;
          uint64_t res, *base;
          unsigned u, *last;
  
          critical_enter();
          base = DPCPU_PTR(tc_cpu_ticks_base);
          last = DPCPU_PTR(tc_cpu_ticks_last);
          tc = timehands->th_counter;
          u = tc->tc_get_timecount(tc) & tc->tc_counter_mask;
          if (u < *last)
                  *base += (uint64_t)tc->tc_counter_mask + 1;
          *last = u;
          res = u + *base;
          critical_exit();
          return (res);
  }
  
  void
  cpu_tick_calibration(void)
  {
          static time_t last_calib;
  
          if (time_uptime != last_calib && !(time_uptime & 0xf)) {
                  cpu_tick_calibrate(0);
                  last_calib = time_uptime;
          }
  }
  
  /*
   * This function gets called every 16 seconds on only one designated
   * CPU in the system from hardclock() via cpu_tick_calibration()().
   *
   * Whenever the real time clock is stepped we get called with reset=1
   * to make sure we handle suspend/resume and similar events correctly.
   */
  
  static void
  cpu_tick_calibrate(int reset)
  {
          static uint64_t c_last;
          uint64_t c_this, c_delta;
          static struct bintime  t_last;
          struct bintime t_this, t_delta;
          uint32_t divi;
  
          if (reset) {
                  /* The clock was stepped, abort & reset */
                  t_last.sec = 0;
                  return;
          }
  
          /* we don't calibrate fixed rate cputicks */
          if (!cpu_tick_variable)
                  return;
  
          getbinuptime(&t_this);
          c_this = cpu_ticks();
          if (t_last.sec != 0) {
                  c_delta = c_this - c_last;
                  t_delta = t_this;
                  bintime_sub(&t_delta, &t_last);
                  /*
                   * Headroom:
                   *      2^(64-20) / 16[s] =
                   *      2^(44) / 16[s] =
                   *      17.592.186.044.416 / 16 =
                   *      1.099.511.627.776 [Hz]
                   */
                  divi = t_delta.sec << 20;
                  divi |= t_delta.frac >> (64 - 20);
                  c_delta <<= 20;
                  c_delta /= divi;
                  if (c_delta > cpu_tick_frequency) {
                          if (0 && bootverbose)
                                  printf("cpu_tick increased to %ju Hz\n",
                                      c_delta);
                          cpu_tick_frequency = c_delta;
                  }
          }
          c_last = c_this;
          t_last = t_this;
  }
  
  void
  set_cputicker(cpu_tick_f *func, uint64_t freq, bool isvariable)
  {
  
          if (func == NULL) {
                  cpu_ticks = tc_cpu_ticks;
          } else {
                  cpu_tick_frequency = freq;
                  cpu_tick_variable = isvariable;
                  cpu_ticks = func;
          }
  }
  
  uint64_t
  cpu_tickrate(void)
  {
  
          if (cpu_ticks == tc_cpu_ticks) 
                  return (tc_getfrequency());
          return (cpu_tick_frequency);
  }
  
  /*
   * We need to be slightly careful converting cputicks to microseconds.
   * There is plenty of margin in 64 bits of microseconds (half a million
   * years) and in 64 bits at 4 GHz (146 years), but if we do a multiply
   * before divide conversion (to retain precision) we find that the
   * margin shrinks to 1.5 hours (one millionth of 146y).
   */
  
  uint64_t
  cputick2usec(uint64_t tick)
  {
          uint64_t tr;
          tr = cpu_tickrate();
          return ((tick / tr) * 1000000ULL) + ((tick % tr) * 1000000ULL) / tr;
  }
  
  cpu_tick_f      *cpu_ticks = tc_cpu_ticks;
  
  static int vdso_th_enable = 1;
  static int
  sysctl_fast_gettime(SYSCTL_HANDLER_ARGS)
  {
          int old_vdso_th_enable, error;
  
          old_vdso_th_enable = vdso_th_enable;
          error = sysctl_handle_int(oidp, &old_vdso_th_enable, 0, req);
          if (error != 0)
                  return (error);
          vdso_th_enable = old_vdso_th_enable;
          return (0);
  }
  SYSCTL_PROC(_kern_timecounter, OID_AUTO, fast_gettime,
      CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE,
      NULL, 0, sysctl_fast_gettime, "I", "Enable fast time of day");
  
  uint32_t
  tc_fill_vdso_timehands(struct vdso_timehands *vdso_th)
  {
          struct timehands *th;
          uint32_t enabled;
  
          th = timehands;
          vdso_th->th_scale = th->th_scale;
          vdso_th->th_offset_count = th->th_offset_count;
          vdso_th->th_counter_mask = th->th_counter->tc_counter_mask;
          vdso_th->th_offset = th->th_offset;
          vdso_th->th_boottime = th->th_boottime;
          if (th->th_counter->tc_fill_vdso_timehands != NULL) {
                  enabled = th->th_counter->tc_fill_vdso_timehands(vdso_th,
                      th->th_counter);
          } else
                  enabled = 0;
          if (!vdso_th_enable)
                  enabled = 0;
          return (enabled);
  }
  
  #ifdef COMPAT_FREEBSD32
  uint32_t
  tc_fill_vdso_timehands32(struct vdso_timehands32 *vdso_th32)
  {
          struct timehands *th;
          uint32_t enabled;
  
          th = timehands;
          *(uint64_t *)&vdso_th32->th_scale[0] = th->th_scale;
          vdso_th32->th_offset_count = th->th_offset_count;
          vdso_th32->th_counter_mask = th->th_counter->tc_counter_mask;
          vdso_th32->th_offset.sec = th->th_offset.sec;
          *(uint64_t *)&vdso_th32->th_offset.frac[0] = th->th_offset.frac;
          vdso_th32->th_boottime.sec = th->th_boottime.sec;
          *(uint64_t *)&vdso_th32->th_boottime.frac[0] = th->th_boottime.frac;
          if (th->th_counter->tc_fill_vdso_timehands32 != NULL) {
                  enabled = th->th_counter->tc_fill_vdso_timehands32(vdso_th32,
                      th->th_counter);
          } else
                  enabled = 0;
          if (!vdso_th_enable)
                  enabled = 0;
          return (enabled);
  }
  #endif
  
  #include "opt_ddb.h"
  #ifdef DDB
  #include <ddb/ddb.h>
  
  DB_SHOW_COMMAND(timecounter, db_show_timecounter)
  {
          struct timehands *th;
          struct timecounter *tc;
          u_int val1, val2;
  
          th = timehands;
          tc = th->th_counter;
          val1 = tc->tc_get_timecount(tc);
          __compiler_membar();
          val2 = tc->tc_get_timecount(tc);
  
          db_printf("timecounter %p %s\n", tc, tc->tc_name);
          db_printf("  mask %#x freq %ju qual %d flags %#x priv %p\n",
              tc->tc_counter_mask, (uintmax_t)tc->tc_frequency, tc->tc_quality,
              tc->tc_flags, tc->tc_priv);
          db_printf("  val %#x %#x\n", val1, val2);
          db_printf("timehands adj %#jx scale %#jx ldelta %d off_cnt %d gen %d\n",
              (uintmax_t)th->th_adjustment, (uintmax_t)th->th_scale,
              th->th_large_delta, th->th_offset_count, th->th_generation);
          db_printf("  offset %jd %jd boottime %jd %jd\n",
              (intmax_t)th->th_offset.sec, (uintmax_t)th->th_offset.frac,
              (intmax_t)th->th_boottime.sec, (uintmax_t)th->th_boottime.frac);
  }
  #endif

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