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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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56
kernel/kernel/Processor.cpp
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@@ -289,13 +289,11 @@ namespace Kernel
}
}
void Processor::initialize_tsc(uint8_t shift, uint64_t mult, uint64_t realtime_seconds)
void Processor::initialize_tsc(uint64_t realtime_seconds)
{
auto& shared_page = Processor::shared_page();
shared_page.gettime_shared.shift = shift;
shared_page.gettime_shared.mult = mult;
shared_page.gettime_shared.realtime_seconds = realtime_seconds;
shared_page.gettime_shared.realtime_s = realtime_seconds;
shared_page.gettime_shared.realtime_ns = 0;
update_tsc();
@@ -322,23 +320,57 @@ namespace Kernel
void Processor::update_tsc()
{
auto& sgettime = shared_page().cpus[current_index()].gettime_local;
sgettime.seq = sgettime.seq + 1;
sgettime.last_ns = SystemTimer::get().ns_since_boot_no_tsc();
sgettime.last_tsc = __builtin_ia32_rdtsc();
sgettime.seq = sgettime.seq + 1;
auto& lgettime = shared_page().cpus[current_index()].gettime_local;
lgettime.seq = lgettime.seq + 1;
if (lgettime.seq == 1)
{
const auto tsc_info = SystemTimer::get().tsc_info();
lgettime.shift = tsc_info.shift;
lgettime.mult = tsc_info.mult;
lgettime.last_ns = SystemTimer::get().ns_since_boot_no_tsc();
lgettime.last_tsc = __builtin_ia32_rdtsc();
}
else
{
const auto current_ns = SystemTimer::get().ns_since_boot_no_tsc();
const auto current_tsc = __builtin_ia32_rdtsc();
auto delta_ns = current_tsc - lgettime.last_tsc;
if (lgettime.shift >= 0)
delta_ns <<= lgettime.shift;
else
delta_ns >>= -lgettime.shift;
delta_ns = (delta_ns * lgettime.mult) >> 32;
lgettime.last_ns += delta_ns;
lgettime.last_tsc = current_tsc;
// scale mult by [-0.25%, 0.25%] to fix for clock drift
const auto error_ns = static_cast<int64_t>(current_ns) - static_cast<int64_t>(lgettime.last_ns);
const auto correction_ppm = BAN::Math::clamp<int64_t>(error_ns * 1'000'000 / 1'000'000'000, -100, 100);
const auto correction_delta = -lgettime.mult * correction_ppm / 1'000'000;
lgettime.mult += correction_delta;
}
lgettime.seq = lgettime.seq + 1;
}
uint64_t Processor::ns_since_boot_tsc()
{
const auto& shared_page = Processor::shared_page();
const auto& sgettime = shared_page.gettime_shared;
const auto& lgettime = shared_page.cpus[current_index()].gettime_local;
auto state = get_interrupt_state();
set_interrupt_state(InterruptState::Disabled);
const auto current_ns = lgettime.last_ns + (((__builtin_ia32_rdtsc() - lgettime.last_tsc) * sgettime.mult) >> sgettime.shift);
uint64_t current_ns = __builtin_ia32_rdtsc() - lgettime.last_tsc;
if (lgettime.shift >= 0)
current_ns <<= lgettime.shift;
else
current_ns >>= -lgettime.shift;
current_ns = (current_ns * lgettime.mult) >> 32;
current_ns += lgettime.last_ns;
set_interrupt_state(state);