Kernel: Stop idling after handling an IRQ

If we receive an IRQ while the idle task is running, prevent it from
re-halting the CPU after the IRQ handler returns.

Instead have the idle task yield to the scheduler, so we can see if
the IRQ has unblocked something.

Kernel/Arch/i386/CPU.cpp

@@ -490,8 +490,10 @@ void handle_irq()
         }
     }
 
-    if (s_irq_handler[irq])
+    if (s_irq_handler[irq]) {
         s_irq_handler[irq]->handle_irq();
+        Scheduler::stop_idling();
+    }
     PIC::eoi(irq);
 }
 

Kernel/Scheduler.cpp

@@ -575,3 +575,24 @@ void Scheduler::timer_tick(RegisterDump& regs)
         "orl $0x00004000, (%esp)\n"
         "popf\n");
 }
+
+static bool s_should_stop_idling = false;
+
+void Scheduler::stop_idling()
+{
+    if (current != &s_colonel_process->main_thread())
+        return;
+
+    s_should_stop_idling = true;
+}
+
+void Scheduler::idle_loop()
+{
+    for (;;) {
+        asm("hlt");
+        if (s_should_stop_idling) {
+            s_should_stop_idling = false;
+            yield();
+        }
+    }
+}

Kernel/Scheduler.h

@@ -30,6 +30,8 @@ public:
     static Process* colonel();
     static bool is_active();
     static void beep();
+    static void idle_loop();
+    static void stop_idling();
 
     template<typename Callback>
     static inline IterationDecision for_each_runnable(Callback);

Kernel/init.cpp

@@ -277,8 +277,6 @@ extern "C" [[noreturn]] void init()
 
     sti();
 
-    // This now becomes the idle process :^)
+    Scheduler::idle_loop();
-    for (;;) {
+    ASSERT_NOT_REACHED();
-        asm("hlt");
-    }
 }