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 /*
  45  * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
  46  * yielding a 64-bit result.
  47  */
  48 static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
  49 {
  50         u64 product;
  51 #ifdef __i386__
  52         u32 tmp1, tmp2;
  53 #endif
  54
  55         if (shift < 0)
  56                 delta >>= -shift;
  57         else
  58                 delta <<= shift;
  59
  60 #ifdef __i386__
  61         __asm__ (
  62                 "mul  %5       ; "
  63                 "mov  %4,%%eax ; "
  64                 "mov  %%edx,%4 ; "
  65                 "mul  %5       ; "
  66                 "xor  %5,%5    ; "
  67                 "add  %4,%%eax ; "
  68                 "adc  %5,%%edx ; "
  69                 : "=A" (product), "=r" (tmp1), "=r" (tmp2)
  70                 : "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (mul_frac) );
  71 #elif defined(__x86_64__)
  72         __asm__ (
  73                 "mul %%rdx ; shrd $32,%%rdx,%%rax"
  74                 : "=a" (product) : "0" (delta), "d" ((u64)mul_frac) );
  75 #else
  76 #error implement me!
  77 #endif
  78
  79         return product;
  80 }
  81
  82 static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
  83 {
  84         u64 delta = native_read_tsc() - shadow->tsc_timestamp;
  85         return scale_delta(delta, shadow->tsc_to_nsec_mul, shadow->tsc_shift);
  86 }
  87
  88 /*
  89  * Reads a consistent set of time-base values from hypervisor,
  90  * into a shadow data area.
  91  */
  92 static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
  93                                         struct pvclock_vcpu_time_info *src)
  94 {
  95         do {
  96                 dst->version = src->version;
  97                 rmb();          /* fetch version before data */
  98                 dst->tsc_timestamp     = src->tsc_timestamp;
  99                 dst->system_timestamp  = src->system_time;
 100                 dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
 101                 dst->tsc_shift         = src->tsc_shift;
 102                 dst->flags             = src->flags;
 103                 rmb();          /* test version after fetching data */
 104         } while ((src->version & 1) || (dst->version != src->version));
 105
 106         return dst->version;
 107 }
 108
 109 unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
 110 {
 111         u64 pv_tsc_khz = 1000000ULL << 32;
 112
 113         do_div(pv_tsc_khz, src->tsc_to_system_mul);
 114         if (src->tsc_shift < 0)
 115                 pv_tsc_khz <<= -src->tsc_shift;
 116         else
 117                 pv_tsc_khz >>= src->tsc_shift;
 118         return pv_tsc_khz;
 119 }
 120
 121 static atomic64_t last_value = ATOMIC64_INIT(0);
 122
 123 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 124 {
 125         struct pvclock_shadow_time shadow;
 126         unsigned version;
 127         cycle_t ret, offset;
 128         u64 last;
 129
 130         do {
 131                 version = pvclock_get_time_values(&shadow, src);
 132                 barrier();
 133                 offset = pvclock_get_nsec_offset(&shadow);
 134                 ret = shadow.system_timestamp + offset;
 135                 barrier();
 136         } while (version != src->version);
 137
 138         if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
 139                 (shadow.flags & PVCLOCK_TSC_STABLE_BIT))
 140                 return ret;
 141
 142         /*
 143          * Assumption here is that last_value, a global accumulator, always goes
 144          * forward. If we are less than that, we should not be much smaller.
 145          * We assume there is an error marging we're inside, and then the correction
 146          * does not sacrifice accuracy.
 147          *
 148          * For reads: global may have changed between test and return,
 149          * but this means someone else updated poked the clock at a later time.
 150          * We just need to make sure we are not seeing a backwards event.
 151          *
 152          * For updates: last_value = ret is not enough, since two vcpus could be
 153          * updating at the same time, and one of them could be slightly behind,
 154          * making the assumption that last_value always go forward fail to hold.
 155          */
 156         last = atomic64_read(&last_value);
 157         do {
 158                 if (ret < last)
 159                         return last;
 160                 last = atomic64_cmpxchg(&last_value, last, ret);
 161         } while (unlikely(last != ret));
 162
 163         return ret;
 164 }
 165
 166 void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
 167                             struct pvclock_vcpu_time_info *vcpu_time,
 168                             struct timespec *ts)
 169 {
 170         u32 version;
 171         u64 delta;
 172         struct timespec now;
 173
 174         /* get wallclock at system boot */
 175         do {
 176                 version = wall_clock->version;
 177                 rmb();          /* fetch version before time */
 178                 now.tv_sec  = wall_clock->sec;
 179                 now.tv_nsec = wall_clock->nsec;
 180                 rmb();          /* fetch time before checking version */
 181         } while ((wall_clock->version & 1) || (version != wall_clock->version));
 182
 183         delta = pvclock_clocksource_read(vcpu_time);    /* time since system boot */
 184         delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
 185
 186         now.tv_nsec = do_div(delta, NSEC_PER_SEC);
 187         now.tv_sec = delta;
 188
 189         set_normalized_timespec(ts, now.tv_sec, now.tv_nsec);
 190 }