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 };
  35
  36 /*
  37  * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
  38  * yielding a 64-bit result.
  39  */
  40 static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
  41 {
  42         u64 product;
  43 #ifdef __i386__
  44         u32 tmp1, tmp2;
  45 #endif
  46
  47         if (shift < 0)
  48                 delta >>= -shift;
  49         else
  50                 delta <<= shift;
  51
  52 #ifdef __i386__
  53         __asm__ (
  54                 "mul  %5       ; "
  55                 "mov  %4,%%eax ; "
  56                 "mov  %%edx,%4 ; "
  57                 "mul  %5       ; "
  58                 "xor  %5,%5    ; "
  59                 "add  %4,%%eax ; "
  60                 "adc  %5,%%edx ; "
  61                 : "=A" (product), "=r" (tmp1), "=r" (tmp2)
  62                 : "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (mul_frac) );
  63 #elif defined(__x86_64__)
  64         __asm__ (
  65                 "mul %%rdx ; shrd $32,%%rdx,%%rax"
  66                 : "=a" (product) : "0" (delta), "d" ((u64)mul_frac) );
  67 #else
  68 #error implement me!
  69 #endif
  70
  71         return product;
  72 }
  73
  74 static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
  75 {
  76         u64 delta = native_read_tsc() - shadow->tsc_timestamp;
  77         return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
  78                                    shadow->tsc_shift);
  79 }
  80
  81 /*
  82  * Reads a consistent set of time-base values from hypervisor,
  83  * into a shadow data area.
  84  */
  85 static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
  86                                         struct pvclock_vcpu_time_info *src)
  87 {
  88         do {
  89                 dst->version = src->version;
  90                 rmb();          /* fetch version before data */
  91                 dst->tsc_timestamp     = src->tsc_timestamp;
  92                 dst->system_timestamp  = src->system_time;
  93                 dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
  94                 dst->tsc_shift         = src->tsc_shift;
  95                 rmb();          /* test version after fetching data */
  96         } while ((src->version & 1) || (dst->version != src->version));
  97
  98         return dst->version;
  99 }
 100
 101 unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
 102 {
 103         u64 pv_tsc_khz = 1000000ULL << 32;
 104
 105         do_div(pv_tsc_khz, src->tsc_to_system_mul);
 106         if (src->tsc_shift < 0)
 107                 pv_tsc_khz <<= -src->tsc_shift;
 108         else
 109                 pv_tsc_khz >>= src->tsc_shift;
 110         return pv_tsc_khz;
 111 }
 112
 113 static atomic64_t last_value = ATOMIC64_INIT(0);
 114
 115 void pvclock_resume(void)
 116 {
 117         atomic64_set(&last_value, 0);
 118 }
 119
 120 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 121 {
 122         struct pvclock_shadow_time shadow;
 123         unsigned version;
 124         cycle_t ret, offset;
 125         u64 last;
 126
 127         do {
 128                 version = pvclock_get_time_values(&shadow, src);
 129                 barrier();
 130                 offset = pvclock_get_nsec_offset(&shadow);
 131                 ret = shadow.system_timestamp + offset;
 132                 barrier();
 133         } while (version != src->version);
 134
 135         /*
 136          * Assumption here is that last_value, a global accumulator, always goes
 137          * forward. If we are less than that, we should not be much smaller.
 138          * We assume there is an error marging we're inside, and then the correction
 139          * does not sacrifice accuracy.
 140          *
 141          * For reads: global may have changed between test and return,
 142          * but this means someone else updated poked the clock at a later time.
 143          * We just need to make sure we are not seeing a backwards event.
 144          *
 145          * For updates: last_value = ret is not enough, since two vcpus could be
 146          * updating at the same time, and one of them could be slightly behind,
 147          * making the assumption that last_value always go forward fail to hold.
 148          */
 149         last = atomic64_read(&last_value);
 150         do {
 151                 if (ret < last)
 152                         return last;
 153                 last = atomic64_cmpxchg(&last_value, last, ret);
 154         } while (unlikely(last != ret));
 155
 156         return ret;
 157 }
 158
 159 void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
 160                             struct pvclock_vcpu_time_info *vcpu_time,
 161                             struct timespec *ts)
 162 {
 163         u32 version;
 164         u64 delta;
 165         struct timespec now;
 166
 167         /* get wallclock at system boot */
 168         do {
 169                 version = wall_clock->version;
 170                 rmb();          /* fetch version before time */
 171                 now.tv_sec  = wall_clock->sec;
 172                 now.tv_nsec = wall_clock->nsec;
 173                 rmb();          /* fetch time before checking version */
 174         } while ((wall_clock->version & 1) || (version != wall_clock->version));
 175
 176         delta = pvclock_clocksource_read(vcpu_time);    /* time since system boot */
 177         delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
 178
 179         now.tv_nsec = do_div(delta, NSEC_PER_SEC);
 180         now.tv_sec = delta;
 181
 182         set_normalized_timespec(ts, now.tv_sec, now.tv_nsec);
 183 }