Kernel: Fix APIC timer calibration to be more accurate

We were calibrating it to 260 instead of 250 ticks per second (being
off by one for the 1/10th second calibration time), resulting in
ticks of only ~3.6 ms instead of ~4ms. This gets us closer to ~4ms,
but because the APIC isn't nearly as precise as e.g. HPET, it will
only be a best effort. Then, use the higher precision reference
timer to more accurately calculate how many ticks we actually get
each second.

Also the frequency calculation was off, causing a "Frequency too slow"
error with VMware.

Fixes some problems observed in #5539

Kernel/Random.cpp

@@ -71,7 +71,7 @@ UNMAP_AFTER_INIT KernelRng::KernelRng()
         klog() << "KernelRng: Using HPET as entropy source";
 
         for (size_t i = 0; i < resource().pool_count * resource().reseed_threshold; ++i) {
-            u64 hpet_time = HPET::the().read_main_counter();
+            u64 hpet_time = HPET::the().read_main_counter_unsafe();
             this->resource().add_random_event(hpet_time, i % 32);
         }
     } else {

Kernel/Time/APICTimer.cpp

@@ -65,28 +65,33 @@ UNMAP_AFTER_INIT bool APICTimer::calibrate(HardwareTimerBase& calibration_source
 
     // temporarily replace the timer callbacks
     const size_t ticks_in_100ms = calibration_source.ticks_per_second() / 10;
-    Atomic<size_t> calibration_ticks = 0;
+    Atomic<size_t, AK::memory_order_relaxed> calibration_ticks = 0;
 #ifdef APIC_TIMER_MEASURE_CPU_CLOCK
     volatile u64 start_tsc = 0, end_tsc = 0;
 #endif
+    volatile u64 start_reference = 0, end_reference = 0;
     volatile u32 start_apic_count = 0, end_apic_count = 0;
+    bool query_reference = calibration_source.can_query_raw();
     auto original_source_callback = calibration_source.set_callback([&](const RegisterState&) {
         u32 current_timer_count = apic.get_timer_current_count();
 #ifdef APIC_TIMER_MEASURE_CPU_CLOCK
         u64 current_tsc = supports_tsc ? read_tsc() : 0;
 #endif
+        u64 current_reference = query_reference ? calibration_source.current_raw() : 0;
 
-        auto prev_tick = calibration_ticks.fetch_add(1, AK::memory_order_acq_rel);
+        auto prev_tick = calibration_ticks.fetch_add(1);
         if (prev_tick == 0) {
 #ifdef APIC_TIMER_MEASURE_CPU_CLOCK
             start_tsc = current_tsc;
 #endif
             start_apic_count = current_timer_count;
-        } else if (prev_tick + 1 == ticks_in_100ms) {
+            start_reference = current_reference;
+        } else if (prev_tick + 1 == ticks_in_100ms + 1) {
 #ifdef APIC_TIMER_MEASURE_CPU_CLOCK
             end_tsc = current_tsc;
 #endif
             end_apic_count = current_timer_count;
+            end_reference = current_reference;
         }
     });
 
@@ -102,7 +107,7 @@ UNMAP_AFTER_INIT bool APICTimer::calibrate(HardwareTimerBase& calibration_source
 
     sti();
     // Loop for about 100 ms
-    while (calibration_ticks.load(AK::memory_order_relaxed) < ticks_in_100ms)
+    while (calibration_ticks.load() <= ticks_in_100ms)
         ;
     cli();
 
@@ -112,25 +117,33 @@ UNMAP_AFTER_INIT bool APICTimer::calibrate(HardwareTimerBase& calibration_source
 
     disable_local_timer();
 
+    if (query_reference) {
+        u64 one_tick_ns = calibration_source.raw_to_ns((end_reference - start_reference) / ticks_in_100ms);
+        m_frequency = (u32)(1000000000ull / one_tick_ns);
+        klog() << "APICTimer: Ticks per second: " << m_frequency << " (" << (one_tick_ns / 1000000) << "." << (one_tick_ns % 1000000) << "ms)";
+    } else {
+        // For now, assume the frequency is exactly the same
+        m_frequency = calibration_source.ticks_per_second();
+        klog() << "APICTimer: Ticks per second: " << m_frequency << " (assume same frequency as reference clock)";
+    }
+
     auto delta_apic_count = start_apic_count - end_apic_count; // The APIC current count register decrements!
     m_timer_period = (delta_apic_count * apic.get_timer_divisor()) / ticks_in_100ms;
 
-    auto apic_freq = (delta_apic_count * apic.get_timer_divisor()) / apic.get_timer_divisor();
+    u64 apic_freq = delta_apic_count * apic.get_timer_divisor() * 10;
+    klog() << "APICTimer: Bus clock speed: " << (apic_freq / 1000000) << "." << (apic_freq % 1000000) << " MHz";
     if (apic_freq < 1000000) {
         klog() << "APICTimer: Frequency too slow!";
         return false;
     }
-    klog() << "APICTimer: Bus clock speed: " << (apic_freq / 1000000) << "." << (apic_freq % 1000000) << " MHz";
+
 #ifdef APIC_TIMER_MEASURE_CPU_CLOCK
     if (supports_tsc) {
-        auto delta_tsc = end_tsc - start_tsc;
+        auto delta_tsc = (end_tsc - start_tsc) * 10;
         klog() << "APICTimer: CPU clock speed: " << (delta_tsc / 1000000) << "." << (delta_tsc % 1000000) << " MHz";
     }
 #endif
 
-    // TODO: measure rather than assuming it matches?
-    m_frequency = calibration_source.ticks_per_second();
-
     enable_local_timer();
     return true;
 }

Kernel/Time/HPET.cpp

@@ -38,7 +38,7 @@ namespace Kernel {
 
 #define ABSOLUTE_MAXIMUM_COUNTER_TICK_PERIOD 0x05F5E100
 #define NANOSECOND_PERIOD_TO_HERTZ(x) 1000000000 / x
-#define MEGAHERTZ_TO_HERTZ(x) (x / 1000000)
+#define HERTZ_TO_MEGAHERTZ(x) (x / 1000000)
 
 namespace HPETFlags {
 enum class Attributes {
@@ -220,7 +220,7 @@ void HPET::update_periodic_comparator_value()
             // way to resume periodic timers properly because we reset the main counter
             // and we can only write the period into the comparator value...
             timer.capabilities = timer.capabilities | (u32)HPETFlags::TimerConfiguration::ValueSet;
-            u64 value = frequency() / comparator.ticks_per_second();
+            u64 value = ns_to_raw_counter_ticks(1000000000ull / comparator.ticks_per_second());
             dbgln_if(HPET_DEBUG, "HPET: Update periodic comparator {} comparator value to {} main value was: {}",
                 comparator.comparator_number(),
                 value,
@@ -250,11 +250,10 @@ void HPET::update_non_periodic_comparator_value(const HPETComparator& comparator
     VERIFY_INTERRUPTS_DISABLED();
     VERIFY(!comparator.is_periodic());
     VERIFY(comparator.comparator_number() <= m_comparators.size());
-    auto& regs = registers();
+    auto& timer = registers().get_timer_by_index(comparator.comparator_number());
-    auto& timer = regs.get_timer_by_index(comparator.comparator_number());
     u64 value = frequency() / comparator.ticks_per_second();
     // NOTE: If the main counter passes this new value before we finish writing it, we will never receive an interrupt!
-    u64 new_counter_value = read_register_safe64(regs.main_counter_value) + value;
+    u64 new_counter_value = read_main_counter() + value;
     timer.comparator_value.high = (u32)(new_counter_value >> 32);
     timer.comparator_value.low = (u32)new_counter_value;
 }
@@ -262,7 +261,7 @@ void HPET::update_non_periodic_comparator_value(const HPETComparator& comparator
 u64 HPET::update_time(u64& seconds_since_boot, u32& ticks_this_second, bool query_only)
 {
     // Should only be called by the time keeper interrupt handler!
-    u64 current_value = read_register_safe64(registers().main_counter_value);
+    u64 current_value = read_main_counter();
     u64 delta_ticks = m_main_counter_drift;
     if (current_value >= m_main_counter_last_read)
         delta_ticks += current_value - m_main_counter_last_read;
@@ -286,12 +285,17 @@ u64 HPET::update_time(u64& seconds_since_boot, u32& ticks_this_second, bool quer
     return (delta_ticks * 1000000000ull) / ticks_per_second;
 }
 
-u64 HPET::read_main_counter() const
+u64 HPET::read_main_counter_unsafe() const
 {
     auto& main_counter = registers().main_counter_value;
     return ((u64)main_counter.high << 32) | (u64)main_counter.low;
 }
 
+u64 HPET::read_main_counter() const
+{
+    return read_register_safe64(registers().main_counter_value);
+}
+
 void HPET::enable_periodic_interrupt(const HPETComparator& comparator)
 {
 #if HPET_DEBUG
@@ -405,10 +409,15 @@ HPETRegistersBlock& HPET::registers()
     return *(HPETRegistersBlock*)m_hpet_mmio_region->vaddr().offset(m_physical_acpi_hpet_registers.offset_in_page()).as_ptr();
 }
 
-u64 HPET::calculate_ticks_in_nanoseconds() const
+u64 HPET::raw_counter_ticks_to_ns(u64 raw_ticks) const
 {
     // ABSOLUTE_MAXIMUM_COUNTER_TICK_PERIOD == 100 nanoseconds
-    return ((u64)registers().capabilities.main_counter_tick_period * 100ull) / ABSOLUTE_MAXIMUM_COUNTER_TICK_PERIOD;
+    return (raw_ticks * (u64)registers().capabilities.main_counter_tick_period * 100ull) / ABSOLUTE_MAXIMUM_COUNTER_TICK_PERIOD;
+}
+
+u64 HPET::ns_to_raw_counter_ticks(u64 ns) const
+{
+    return (ns * 1000000ull) / (u64)registers().capabilities.main_counter_tick_period;
 }
 
 UNMAP_AFTER_INIT HPET::HPET(PhysicalAddress acpi_hpet)
@@ -438,8 +447,8 @@ UNMAP_AFTER_INIT HPET::HPET(PhysicalAddress acpi_hpet)
 
     global_disable();
 
-    m_frequency = NANOSECOND_PERIOD_TO_HERTZ(calculate_ticks_in_nanoseconds());
+    m_frequency = NANOSECOND_PERIOD_TO_HERTZ(raw_counter_ticks_to_ns(1));
-    klog() << "HPET: frequency " << m_frequency << " Hz (" << MEGAHERTZ_TO_HERTZ(m_frequency) << " MHz) resolution: " << calculate_ticks_in_nanoseconds() << "ns";
+    klog() << "HPET: frequency " << m_frequency << " Hz (" << HERTZ_TO_MEGAHERTZ(m_frequency) << " MHz) resolution: " << raw_counter_ticks_to_ns(1) << "ns";
     VERIFY(regs.capabilities.main_counter_tick_period <= ABSOLUTE_MAXIMUM_COUNTER_TICK_PERIOD);
 
     // Reset the counter, just in case... (needs to match m_main_counter_last_read)

Kernel/Time/HPET.h

@@ -46,6 +46,8 @@ public:
     static HPET& the();
 
     u64 frequency() const { return m_frequency; }
+    u64 raw_counter_ticks_to_ns(u64) const;
+    u64 ns_to_raw_counter_ticks(u64) const;
 
     const NonnullRefPtrVector<HPETComparator>& comparators() const { return m_comparators; }
     void disable(const HPETComparator&);
@@ -60,6 +62,7 @@ public:
     void disable_periodic_interrupt(const HPETComparator& comparator);
 
     u64 update_time(u64& seconds_since_boot, u32& ticks_this_second, bool query_only);
+    u64 read_main_counter_unsafe() const;
     u64 read_main_counter() const;
 
     Vector<unsigned> capable_interrupt_numbers(u8 comparator_number);
@@ -75,7 +78,7 @@ private:
     bool is_periodic_capable(u8 comparator_number);
     void set_comparators_to_optimal_interrupt_state(size_t timers_count);
 
-    u64 calculate_ticks_in_nanoseconds() const;
+    u64 nanoseconds_to_raw_ticks() const;
 
     PhysicalAddress find_acpi_hpet_registers_block();
     explicit HPET(PhysicalAddress acpi_hpet);

Kernel/Time/HPETComparator.cpp

@@ -138,4 +138,14 @@ size_t HPETComparator::calculate_nearest_possible_frequency(size_t frequency) co
     return frequency;
 }
 
+u64 HPETComparator::current_raw() const
+{
+    return HPET::the().read_main_counter();
+}
+
+u64 HPETComparator::raw_to_ns(u64 raw_delta) const
+{
+    return HPET::the().raw_counter_ticks_to_ns(raw_delta);
+}
+
 }

Kernel/Time/HPETComparator.h

@@ -51,6 +51,9 @@ public:
     virtual void set_periodic() override;
     virtual void set_non_periodic() override;
     virtual void disable() override;
+    virtual bool can_query_raw() const override { return true; }
+    virtual u64 current_raw() const override;
+    virtual u64 raw_to_ns(u64) const override;
 
     virtual void reset_to_default_ticks_per_second() override;
     virtual bool try_to_set_frequency(size_t frequency) override;

Kernel/Time/HardwareTimer.h

@@ -65,6 +65,9 @@ public:
     virtual void set_non_periodic() = 0;
     virtual void disable() = 0;
     virtual u32 frequency() const = 0;
+    virtual bool can_query_raw() const { return false; }
+    virtual u64 current_raw() const { return 0; }
+    virtual u64 raw_to_ns(u64) const { return 0; }
 
     virtual size_t ticks_per_second() const = 0;