[zion] Move to using the LAPIC timer over the PIT.

zion/interrupt/apic_timer.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include <stdint.h>
+
+class ApicTimer {
+ public:
+  static void Init();
+
+  // FIXME: The calibration is not currently very accurate due to time taken
+  // handling the PIT interrupts. It would probably be good to revisit this
+  // after implementing HPET.
+  void Calibrate();
+
+  void WaitCalibration() {
+    while (calculated_frequency_ == 0) {
+      asm("hlt;");
+    }
+  }
+
+ private:
+  struct Calibration {
+    uint32_t initial_measurement = 0;
+    uint32_t tick_count = 0;
+  };
+  Calibration calibration_;
+
+  uint64_t calculated_frequency_ = 0;
+
+  ApicTimer() {}
+  void StartCalibration();
+  void FinishCalibration();
+};
+
+extern ApicTimer* gApicTimer;