add a sleep-if-idle implementation

src/task/executor.rs

@@ -21,6 +21,7 @@ impl Executor {
     pub fn run(&mut self) -> ! {
         loop {
             self.run_ready_tasks();
+            self.sleep_if_idle();
         }
     }
 
@@ -32,6 +33,17 @@ impl Executor {
         self.task_queue.push(task_id).expect("queue full");
     }
 
+    fn sleep_if_idle(&self) {
+        use x86_64::instructions::interrupts::{self, enable_and_hlt};
+
+        interrupts::disable();
+        if self.task_queue.is_empty() {
+            enable_and_hlt();
+        } else {
+            interrupts::enable();
+        }
+    }
+
     fn run_ready_tasks(&mut self) {
         // destructure `self` to avoid borrow checker errors
         let Self {