arch/x86/vdso/vclock_gettime.c

   1 /*
   2  * Copyright 2006 Andi Kleen, SUSE Labs.
   3  * Subject to the GNU Public License, v.2
   4  *
   5  * Fast user context implementation of clock_gettime, gettimeofday, and time.
   6  *
   7  * The code should have no internal unresolved relocations.
   8  * Check with readelf after changing.
   9  */
  10
  11 /* Disable profiling for userspace code: */
  12 #define DISABLE_BRANCH_PROFILING
  13
  14 #include <linux/kernel.h>
  15 #include <linux/posix-timers.h>
  16 #include <linux/time.h>
  17 #include <linux/string.h>
  18 #include <asm/vsyscall.h>
  19 #include <asm/fixmap.h>
  20 #include <asm/vgtod.h>
  21 #include <asm/timex.h>
  22 #include <asm/hpet.h>
  23 #include <asm/unistd.h>
  24 #include <asm/io.h>
  25
  26 #define gtod (&VVAR(vsyscall_gtod_data))
  27
  28 notrace static cycle_t vread_tsc(void)
  29 {
  30         cycle_t ret;
  31         u64 last;
  32
  33         /*
  34          * Empirically, a fence (of type that depends on the CPU)
  35          * before rdtsc is enough to ensure that rdtsc is ordered
  36          * with respect to loads.  The various CPU manuals are unclear
  37          * as to whether rdtsc can be reordered with later loads,
  38          * but no one has ever seen it happen.
  39          */
  40         rdtsc_barrier();
  41         ret = (cycle_t)vget_cycles();
  42
  43         last = VVAR(vsyscall_gtod_data).clock.cycle_last;
  44
  45         if (likely(ret >= last))
  46                 return ret;
  47
  48         /*
  49          * GCC likes to generate cmov here, but this branch is extremely
  50          * predictable (it's just a funciton of time and the likely is
  51          * very likely) and there's a data dependence, so force GCC
  52          * to generate a branch instead.  I don't barrier() because
  53          * we don't actually need a barrier, and if this function
  54          * ever gets inlined it will generate worse code.
  55          */
  56         asm volatile ("");
  57         return last;
  58 }
  59
  60 static notrace cycle_t vread_hpet(void)
  61 {
  62         return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
  63 }
  64
  65 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
  66 {
  67         long ret;
  68         asm("syscall" : "=a" (ret) :
  69             "0" (__NR_clock_gettime),"D" (clock), "S" (ts) : "memory");
  70         return ret;
  71 }
  72
  73 notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
  74 {
  75         long ret;
  76
  77         asm("syscall" : "=a" (ret) :
  78             "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
  79         return ret;
  80 }
  81
  82
  83 notrace static inline u64 vgetsns(void)
  84 {
  85         long v;
  86         cycles_t cycles;
  87         if (gtod->clock.vclock_mode == VCLOCK_TSC)
  88                 cycles = vread_tsc();
  89         else if (gtod->clock.vclock_mode == VCLOCK_HPET)
  90                 cycles = vread_hpet();
  91         else
  92                 return 0;
  93         v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask;
  94         return v * gtod->clock.mult;
  95 }
  96
  97 /* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
  98 notrace static int __always_inline do_realtime(struct timespec *ts)
  99 {
 100         unsigned long seq;
 101         u64 ns;
 102         int mode;
 103
 104         ts->tv_nsec = 0;
 105         do {
 106                 seq = read_seqcount_begin(&gtod->seq);
 107                 mode = gtod->clock.vclock_mode;
 108                 ts->tv_sec = gtod->wall_time_sec;
 109                 ns = gtod->wall_time_snsec;
 110                 ns += vgetsns();
 111                 ns >>= gtod->clock.shift;
 112         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 113
 114         timespec_add_ns(ts, ns);
 115         return mode;
 116 }
 117
 118 notrace static int do_monotonic(struct timespec *ts)
 119 {
 120         unsigned long seq;
 121         u64 ns;
 122         int mode;
 123
 124         ts->tv_nsec = 0;
 125         do {
 126                 seq = read_seqcount_begin(&gtod->seq);
 127                 mode = gtod->clock.vclock_mode;
 128                 ts->tv_sec = gtod->monotonic_time_sec;
 129                 ns = gtod->monotonic_time_snsec;
 130                 ns += vgetsns();
 131                 ns >>= gtod->clock.shift;
 132         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 133         timespec_add_ns(ts, ns);
 134
 135         return mode;
 136 }
 137
 138 notrace static int do_realtime_coarse(struct timespec *ts)
 139 {
 140         unsigned long seq;
 141         do {
 142                 seq = read_seqcount_begin(&gtod->seq);
 143                 ts->tv_sec = gtod->wall_time_coarse.tv_sec;
 144                 ts->tv_nsec = gtod->wall_time_coarse.tv_nsec;
 145         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 146         return 0;
 147 }
 148
 149 notrace static int do_monotonic_coarse(struct timespec *ts)
 150 {
 151         unsigned long seq;
 152         do {
 153                 seq = read_seqcount_begin(&gtod->seq);
 154                 ts->tv_sec = gtod->monotonic_time_coarse.tv_sec;
 155                 ts->tv_nsec = gtod->monotonic_time_coarse.tv_nsec;
 156         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 157
 158         return 0;
 159 }
 160
 161 notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 162 {
 163         int ret = VCLOCK_NONE;
 164
 165         switch (clock) {
 166         case CLOCK_REALTIME:
 167                 ret = do_realtime(ts);
 168                 break;
 169         case CLOCK_MONOTONIC:
 170                 ret = do_monotonic(ts);
 171                 break;
 172         case CLOCK_REALTIME_COARSE:
 173                 return do_realtime_coarse(ts);
 174         case CLOCK_MONOTONIC_COARSE:
 175                 return do_monotonic_coarse(ts);
 176         }
 177
 178         if (ret == VCLOCK_NONE)
 179                 return vdso_fallback_gettime(clock, ts);
 180         return 0;
 181 }
 182 int clock_gettime(clockid_t, struct timespec *)
 183         __attribute__((weak, alias("__vdso_clock_gettime")));
 184
 185 notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 186 {
 187         long ret = VCLOCK_NONE;
 188
 189         if (likely(tv != NULL)) {
 190                 BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
 191                              offsetof(struct timespec, tv_nsec) ||
 192                              sizeof(*tv) != sizeof(struct timespec));
 193                 ret = do_realtime((struct timespec *)tv);
 194                 tv->tv_usec /= 1000;
 195         }
 196         if (unlikely(tz != NULL)) {
 197                 /* Avoid memcpy. Some old compilers fail to inline it */
 198                 tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
 199                 tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
 200         }
 201
 202         if (ret == VCLOCK_NONE)
 203                 return vdso_fallback_gtod(tv, tz);
 204         return 0;
 205 }
 206 int gettimeofday(struct timeval *, struct timezone *)
 207         __attribute__((weak, alias("__vdso_gettimeofday")));
 208
 209 /*
 210  * This will break when the xtime seconds get inaccurate, but that is
 211  * unlikely
 212  */
 213 notrace time_t __vdso_time(time_t *t)
 214 {
 215         /* This is atomic on x86_64 so we don't need any locks. */
 216         time_t result = ACCESS_ONCE(VVAR(vsyscall_gtod_data).wall_time_sec);
 217
 218         if (t)
 219                 *t = result;
 220         return result;
 221 }
 222 int time(time_t *t)
 223         __attribute__((weak, alias("__vdso_time")));