Kernel: Make TSC based timer monotonic

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

kernel/include/kernel/API/SharedPage.h

@@ -18,9 +18,8 @@ namespace Kernel::API
 
 		struct
 		{
-			uint8_t shift;
-			uint64_t mult;
-			uint64_t realtime_seconds;
+			uint64_t realtime_s;
+			uint32_t realtime_ns;
 		} gettime_shared;
 
 		struct
@@ -28,6 +27,8 @@ namespace Kernel::API
 			struct
 			{
 				uint32_t seq;
+				uint32_t mult;
+				int8_t shift;
 				uint64_t last_ns;
 				uint64_t last_tsc;
 			} gettime_local;

kernel/include/kernel/CPUID.h

@@ -82,5 +82,6 @@ namespace CPUID
 	bool has_pat();
 	bool has_1gib_pages();
 	bool has_invariant_tsc();
+	uint64_t get_tsc_frequency();
 
 }

kernel/include/kernel/Processor.h

@@ -124,7 +124,7 @@ namespace Kernel
 		static void yield();
 		static Scheduler& scheduler() { return *read_gs_sized<Scheduler*>(offsetof(Processor, m_scheduler)); }
 
-		static void initialize_tsc(uint8_t shift, uint64_t mult, uint64_t realtime_seconds);
+		static void initialize_tsc(uint64_t realtime_seconds);
 		static void update_tsc();
 		static uint64_t ns_since_boot_tsc();
 

kernel/include/kernel/Timer/Timer.h

@@ -30,6 +30,13 @@ namespace Kernel
 
 	class SystemTimer : public Timer
 	{
+	public:
+		struct TSCInfo
+		{
+			int8_t shift;
+			uint32_t mult;
+		};
+
 	public:
 		static void initialize();
 		static SystemTimer& get();
@@ -49,8 +56,10 @@ namespace Kernel
 
 		void dont_invoke_scheduler() { m_timer->m_should_invoke_scheduler = false; }
 
-		void update_tsc() const;
-		uint64_t ns_since_boot_no_tsc() const;
+		void update_tsc();
+		TSCInfo tsc_info() const;
+
+		uint64_t ns_since_boot_no_tsc() const { return m_timer->ns_since_boot(); }
 
 		timespec real_time() const;
 
@@ -59,14 +68,29 @@ namespace Kernel
 
 		void initialize_timers();
 
-		uint64_t get_tsc_frequency() const;
+		void initialize_invariant_tsc();
 
 	private:
+		enum class TSCType
+		{
+			None,
+			Invariant,
+		};
+
 		uint64_t m_boot_time { 0 };
 		BAN::UniqPtr<RTC> m_rtc;
 		BAN::UniqPtr<Timer> m_timer;
-		bool m_has_invariant_tsc { false };
-		mutable uint32_t m_timer_ticks { 0 };
+
+		TSCType m_tsc_type = TSCType::None;
+
+		union {
+			struct {
+				int8_t shift;
+				uint32_t mult;
+			} invariant;
+		} m_tsc_info;
+
+		uint64_t m_tsc_update_ns { 0 };
 	};
 
 }