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 /*
  23  * These are perodically updated
  24  *    xen: magic shared_info page
  25  *    kvm: gpa registered via msr
  26  * and then copied here.
  27  */
  28 struct pvclock_shadow_time {
  29         u64 tsc_timestamp;     /* TSC at last update of time vals.  */
  30         u64 system_timestamp;  /* Time, in nanosecs, since boot.    */
  31         u32 tsc_to_nsec_mul;
  32         int tsc_shift;
  33         u32 version;
  34         u8  flags;
  35 };
  36
  37 static u8 valid_flags __read_mostly = 0;
  38
  39 void pvclock_set_flags(u8 flags)
  40 {
  41         valid_flags = flags;
  42 }
  43
  44 static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
  45 {
  46         u64 delta = native_read_tsc() - shadow->tsc_timestamp;
  47         return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
  48                                    shadow->tsc_shift);
  49 }
  50
  51 /*
  52  * Reads a consistent set of time-base values from hypervisor,
  53  * into a shadow data area.
  54  */
  55 static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
  56                                         struct pvclock_vcpu_time_info *src)
  57 {
  58         do {
  59                 dst->version = src->version;
  60                 rmb();          /* fetch version before data */
  61                 dst->tsc_timestamp     = src->tsc_timestamp;
  62                 dst->system_timestamp  = src->system_time;
  63                 dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
  64                 dst->tsc_shift         = src->tsc_shift;
  65                 dst->flags             = src->flags;
  66                 rmb();          /* test version after fetching data */
  67         } while ((src->version & 1) || (dst->version != src->version));
  68
  69         return dst->version;
  70 }
  71
  72 unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
  73 {
  74         u64 pv_tsc_khz = 1000000ULL << 32;
  75
  76         do_div(pv_tsc_khz, src->tsc_to_system_mul);
  77         if (src->tsc_shift < 0)
  78                 pv_tsc_khz <<= -src->tsc_shift;
  79         else
  80                 pv_tsc_khz >>= src->tsc_shift;
  81         return pv_tsc_khz;
  82 }
  83
  84 static atomic64_t last_value = ATOMIC64_INIT(0);
  85
  86 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
  87 {
  88         struct pvclock_shadow_time shadow;
  89         unsigned version;
  90         cycle_t ret, offset;
  91         u64 last;
  92
  93         do {
  94                 version = pvclock_get_time_values(&shadow, src);
  95                 barrier();
  96                 offset = pvclock_get_nsec_offset(&shadow);
  97                 ret = shadow.system_timestamp + offset;
  98                 barrier();
  99         } while (version != src->version);
 100
 101         if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
 102                 (shadow.flags & PVCLOCK_TSC_STABLE_BIT))
 103                 return ret;
 104
 105         /*
 106          * Assumption here is that last_value, a global accumulator, always goes
 107          * forward. If we are less than that, we should not be much smaller.
 108          * We assume there is an error marging we're inside, and then the correction
 109          * does not sacrifice accuracy.
 110          *
 111          * For reads: global may have changed between test and return,
 112          * but this means someone else updated poked the clock at a later time.
 113          * We just need to make sure we are not seeing a backwards event.
 114          *
 115          * For updates: last_value = ret is not enough, since two vcpus could be
 116          * updating at the same time, and one of them could be slightly behind,
 117          * making the assumption that last_value always go forward fail to hold.
 118          */
 119         last = atomic64_read(&last_value);
 120         do {
 121                 if (ret < last)
 122                         return last;
 123                 last = atomic64_cmpxchg(&last_value, last, ret);
 124         } while (unlikely(last != ret));
 125
 126         return ret;
 127 }
 128
 129 void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
 130                             struct pvclock_vcpu_time_info *vcpu_time,
 131                             struct timespec *ts)
 132 {
 133         u32 version;
 134         u64 delta;
 135         struct timespec now;
 136
 137         /* get wallclock at system boot */
 138         do {
 139                 version = wall_clock->version;
 140                 rmb();          /* fetch version before time */
 141                 now.tv_sec  = wall_clock->sec;
 142                 now.tv_nsec = wall_clock->nsec;
 143                 rmb();          /* fetch time before checking version */
 144         } while ((wall_clock->version & 1) || (version != wall_clock->version));
 145
 146         delta = pvclock_clocksource_read(vcpu_time);    /* time since system boot */
 147         delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
 148
 149         now.tv_nsec = do_div(delta, NSEC_PER_SEC);
 150         now.tv_sec = delta;
 151
 152         set_normalized_timespec(ts, now.tv_sec, now.tv_nsec);
 153 }