clock_gettime-mono.c

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2022 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
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│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/atomic.h"
#include "libc/calls/struct/timespec.h"
#include "libc/cosmo.h"
#include "libc/errno.h"
#include "libc/nexgen32e/rdtsc.h"
#include "libc/nexgen32e/x86feature.h"

/**
 * @fileoverview Fast Monotonic Clock Polyfill for XNU/NT.
 */

static struct {
  atomic_uint once;
  struct timespec base_wall;
  uint64_t base_tick;
} g_mono;

static void sys_clock_gettime_mono_init(void) {
  g_mono.base_wall = timespec_real();
  g_mono.base_tick = rdtsc();
}

int sys_clock_gettime_mono(struct timespec *time) {
  uint64_t nanos;
  uint64_t cycles;
#ifdef __x86_64__
  // intel architecture guarantees that a mapping exists between rdtsc &
  // nanoseconds only if the cpu advertises invariant timestamps support
  if (!X86_HAVE(INVTSC))
    return -EINVAL;
#endif
  cosmo_once(&g_mono.once, sys_clock_gettime_mono_init);
  cycles = rdtsc() - g_mono.base_tick;
  // this is a crude approximation, that's worked reasonably well so far
  // only the kernel knows the actual mapping between rdtsc and nanosecs
  // which we could attempt to measure ourselves using clock_gettime but
  // we'd need to impose 100 ms of startup latency for a guess this good
  nanos = cycles / 3;
  *time = timespec_add(g_mono.base_wall, timespec_fromnanos(nanos));
  return 0;
}