Kernel: Abstract platform-specific current time methods from Scheduler

This change ensures that the scheduler doesn't depend on a platform
specific or arch-specific code when it initializes itself, but rather we
ensure that in compile-time we will generate the appropriate code to
find the correct arch-specific current time methods.

Kernel/Arch/CurrentTime.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2022, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+namespace Kernel {
+
+typedef u64 (*fptr)();
+
+fptr optional_current_time();
+
+}

Kernel/Arch/aarch64/CurrentTime.cpp

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2022, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <Kernel/Arch/CurrentTime.h>
+
+namespace Kernel {
+
+fptr optional_current_time()
+{
+    return nullptr;
+}
+
+}

Kernel/Arch/x86/CurrentTime.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/CurrentTime.h>
+#include <Kernel/Arch/x86/ASM_wrapper.h>
+#include <Kernel/Arch/x86/Processor.h>
+
+namespace Kernel {
+
+static u64 current_time_tsc()
+{
+    return read_tsc();
+}
+
+fptr optional_current_time()
+{
+    VERIFY(Processor::is_initialized()); // sanity check
+    // Figure out a good scheduling time source
+    if (Processor::current().has_feature(CPUFeature::TSC) && Processor::current().has_feature(CPUFeature::CONSTANT_TSC)) {
+        return current_time_tsc;
+    }
+    return nullptr;
+}
+
+}

Kernel/CMakeLists.txt

@@ -327,6 +327,8 @@ if ("${SERENITY_ARCH}" STREQUAL "i686" OR "${SERENITY_ARCH}" STREQUAL "x86_64")
         Arch/x86/common/SmapDisabler.cpp
         Arch/x86/common/Shutdown.cpp
 
+        Arch/x86/CurrentTime.cpp
+
         Arch/x86/Hypervisor/BochsDisplayConnector.cpp
         Arch/x86/Hypervisor/VMWareBackdoor.cpp
 
@@ -477,6 +479,7 @@ else()
         Arch/aarch64/boot.S
         Arch/aarch64/BootPPMParser.cpp
         Arch/aarch64/CrashHandler.cpp
+        Arch/aarch64/CurrentTime.cpp
         Arch/aarch64/Dummy.cpp
         Arch/aarch64/Exceptions.cpp
         Arch/aarch64/init.cpp

Kernel/Scheduler.cpp

@@ -8,6 +8,7 @@
 #include <AK/ScopeGuard.h>
 #include <AK/Singleton.h>
 #include <AK/Time.h>
+#include <Kernel/Arch/CurrentTime.h>
 #include <Kernel/Arch/InterruptDisabler.h>
 #include <Kernel/Arch/x86/TrapFrame.h>
 #include <Kernel/Debug.h>
@@ -365,11 +366,6 @@ Process* Scheduler::colonel()
     return s_colonel_process;
 }
 
-static u64 current_time_tsc()
-{
-    return read_tsc();
-}
-
 static u64 current_time_monotonic()
 {
     // We always need a precise timestamp here, we cannot rely on a coarse timestamp
@@ -380,13 +376,11 @@ UNMAP_AFTER_INIT void Scheduler::initialize()
 {
     VERIFY(Processor::is_initialized()); // sanity check
 
-    // Figure out a good scheduling time source
-    if (Processor::current().has_feature(CPUFeature::TSC) && Processor::current().has_feature(CPUFeature::CONSTANT_TSC)) {
-        current_time = current_time_tsc;
-    } else {
-        // TODO: Using HPET is rather slow, can we use any other time source that may be faster?
+    auto* possible_arch_specific_current_time_function = optional_current_time();
+    if (possible_arch_specific_current_time_function)
+        current_time = possible_arch_specific_current_time_function;
+    else
         current_time = current_time_monotonic;
-    }
 
     LockRefPtr<Thread> idle_thread;
     g_finalizer_wait_queue = new WaitQueue;