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https://github.com/jart/cosmopolitan.git
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80 lines
3.8 KiB
C
80 lines
3.8 KiB
C
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi │
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2022 Justine Alexandra Roberts Tunney │
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│ │
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│ Permission to use, copy, modify, and/or distribute this software for │
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│ any purpose with or without fee is hereby granted, provided that the │
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│ above copyright notice and this permission notice appear in all copies. │
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│ │
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│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL │
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│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED │
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│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE │
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│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL │
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│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR │
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│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER │
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│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
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│ PERFORMANCE OF THIS SOFTWARE. │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/atomic.h"
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#include "libc/calls/struct/timespec.h"
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#include "libc/calls/struct/timespec.internal.h"
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#include "libc/calls/struct/timeval.h"
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#include "libc/cosmo.h"
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#include "libc/dce.h"
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#include "libc/nexgen32e/rdtsc.h"
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/**
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* @fileoverview Monotonic clock polyfill.
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*
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* This isn't quite `CLOCK_MONOTONIC` and isn't quite `CLOCK_BOOTTIME`
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* either; however it is fast and almost always goes in one direction.
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*
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* Intel architecture guarantees that a mapping exists between rdtsc &
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* nanoseconds only if the cpu advertises invariant timestamps support
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* however this shouldn't matter for a monotonic clock since we really
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* don't want to have it tick while suspended. Sadly that shall happen
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* since nearly all x86 microprocessors support invariant tsc which is
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* why we try to avoid this fallback when possible.
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*/
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int sys_sysctl(int *, unsigned, void *, size_t *, void *, size_t) libcesque;
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static struct {
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atomic_uint once;
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unsigned long base;
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struct timespec boot;
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} g_mono;
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static struct timespec get_boot_time_xnu(void) {
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struct timeval t;
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size_t n = sizeof(t);
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int mib[] = {1 /* CTL_KERN */, 21 /* KERN_BOOTTIME */};
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if (sys_sysctl(mib, 2, &t, &n, 0, 0) == -1)
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__builtin_trap();
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return timeval_totimespec(t);
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}
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static void sys_clock_gettime_mono_init(void) {
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g_mono.base = rdtsc();
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if (IsXnu()) {
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g_mono.boot = get_boot_time_xnu();
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} else {
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__builtin_trap();
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}
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}
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int sys_clock_gettime_mono(struct timespec *time) {
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uint64_t nanos;
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uint64_t cycles;
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cosmo_once(&g_mono.once, sys_clock_gettime_mono_init);
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// ensure we get the full 64 bits of counting, which avoids wraparound
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cycles = rdtsc() - g_mono.base;
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// this is a crude approximation, that's worked reasonably well so far
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// only the kernel knows the actual mapping between rdtsc and nanosecs
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// which we could attempt to measure ourselves using clock_gettime but
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// we'd need to impose 100 ms of startup latency for a guess this good
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nanos = cycles / 3;
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*time = timespec_add(g_mono.boot, timespec_fromnanos(nanos));
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return 0;
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}
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