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Kernel: Make TSC based timer monotonic

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Prior to this I was just blindly rebasing the current TSC stats every
once in a while. Now I check for drift and scale the multiplier
accordingly to keep the timer from drifting
This commit is contained in:
Bananymous
2026-06-26 01:32:04 +03:00
parent ac0ef53e87
commit 59ec05c898
8 changed files with 172 additions and 75 deletions
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18
userspace/libraries/LibC/time.cpp
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@@ -46,7 +46,13 @@ int clock_gettime(clockid_t clock_id, struct timespec* tp)
if (old_seq & 1)
continue;
const auto monotonic_ns = lgettime.last_ns + (((__builtin_ia32_rdtsc() - lgettime.last_tsc) * sgettime.mult) >> sgettime.shift);
uint64_t monotonic_ns = __builtin_ia32_rdtsc() - lgettime.last_tsc;
if (lgettime.shift >= 0)
monotonic_ns <<= lgettime.shift;
else
monotonic_ns >>= -lgettime.shift;
monotonic_ns = (monotonic_ns * lgettime.mult) >> 32;
monotonic_ns += lgettime.last_ns;
if (old_seq != lgettime.seq || cpu != get_cpu())
continue;
@@ -57,7 +63,15 @@ int clock_gettime(clockid_t clock_id, struct timespec* tp)
};
if (clock_id == CLOCK_REALTIME)
tp->tv_sec += sgettime.realtime_seconds;
{
tp->tv_sec += sgettime.realtime_s;
tp->tv_nsec += sgettime.realtime_ns;
if (tp->tv_nsec >= 1'000'000'000)
{
tp->tv_sec += tp->tv_nsec / 1'000'000'000;
tp->tv_nsec = tp->tv_nsec % 1'000'000'000;
}
}
return 0;
}