arch/x86/kernel/pvclock.c

   1 /*  paravirtual clock -- common code used by kvm/xen
   2
   3     This program is free software; you can redistribute it and/or modify
   4     it under the terms of the GNU General Public License as published by
   5     the Free Software Foundation; either version 2 of the License, or
   6     (at your option) any later version.
   7
   8     This program is distributed in the hope that it will be useful,
   9     but WITHOUT ANY WARRANTY; without even the implied warranty of
  10     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11     GNU General Public License for more details.
  12
  13     You should have received a copy of the GNU General Public License
  14     along with this program; if not, write to the Free Software
  15     Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  16 */
  17
  18 #include <linux/kernel.h>
  19 #include <linux/percpu.h>
  20 #include <asm/pvclock.h>
  21
  22 static u8 valid_flags __read_mostly = 0;
  23
  24 void pvclock_set_flags(u8 flags)
  25 {
  26         valid_flags = flags;
  27 }
  28
  29 static u64 pvclock_get_nsec_offset(const struct pvclock_vcpu_time_info *src)
  30 {
  31         u64 delta = native_read_tsc() - src->tsc_timestamp;
  32         return pvclock_scale_delta(delta, src->tsc_to_system_mul,
  33                                    src->tsc_shift);
  34 }
  35
  36 unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
  37 {
  38         u64 pv_tsc_khz = 1000000ULL << 32;
  39
  40         do_div(pv_tsc_khz, src->tsc_to_system_mul);
  41         if (src->tsc_shift < 0)
  42                 pv_tsc_khz <<= -src->tsc_shift;
  43         else
  44                 pv_tsc_khz >>= src->tsc_shift;
  45         return pv_tsc_khz;
  46 }
  47
  48 static atomic64_t last_value = ATOMIC64_INIT(0);
  49
  50 void pvclock_resume(void)
  51 {
  52         atomic64_set(&last_value, 0);
  53 }
  54
  55 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
  56 {
  57         unsigned version;
  58         cycle_t ret, offset;
  59         u64 last;
  60         u8 flags;
  61
  62         do {
  63                 version = src->version;
  64                 rdtsc_barrier();
  65                 offset = pvclock_get_nsec_offset(src);
  66                 ret = src->system_time + offset;
  67                 flags = src->flags;
  68                 rdtsc_barrier();
  69         } while ((src->version & 1) || version != src->version);
  70
  71         if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
  72                 (flags & PVCLOCK_TSC_STABLE_BIT))
  73                 return ret;
  74
  75         /*
  76          * Assumption here is that last_value, a global accumulator, always goes
  77          * forward. If we are less than that, we should not be much smaller.
  78          * We assume there is an error marging we're inside, and then the correction
  79          * does not sacrifice accuracy.
  80          *
  81          * For reads: global may have changed between test and return,
  82          * but this means someone else updated poked the clock at a later time.
  83          * We just need to make sure we are not seeing a backwards event.
  84          *
  85          * For updates: last_value = ret is not enough, since two vcpus could be
  86          * updating at the same time, and one of them could be slightly behind,
  87          * making the assumption that last_value always go forward fail to hold.
  88          */
  89         last = atomic64_read(&last_value);
  90         do {
  91                 if (ret < last)
  92                         return last;
  93                 last = atomic64_cmpxchg(&last_value, last, ret);
  94         } while (unlikely(last != ret));
  95
  96         return ret;
  97 }
  98
  99 void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
 100                             struct pvclock_vcpu_time_info *vcpu_time,
 101                             struct timespec *ts)
 102 {
 103         u32 version;
 104         u64 delta;
 105         struct timespec now;
 106
 107         /* get wallclock at system boot */
 108         do {
 109                 version = wall_clock->version;
 110                 rmb();          /* fetch version before time */
 111                 now.tv_sec  = wall_clock->sec;
 112                 now.tv_nsec = wall_clock->nsec;
 113                 rmb();          /* fetch time before checking version */
 114         } while ((wall_clock->version & 1) || (version != wall_clock->version));
 115
 116         delta = pvclock_clocksource_read(vcpu_time);    /* time since system boot */
 117         delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
 118
 119         now.tv_nsec = do_div(delta, NSEC_PER_SEC);
 120         now.tv_sec = delta;
 121
 122         set_normalized_timespec(ts, now.tv_sec, now.tv_nsec);
 123 }