Prevent 100% CPU utilization by QEMU when running xv6 OS.

xv6-rev11.patch

Patch for the xv6 operating system (version: xv6-rev11).
Prevents 100% CPU utilization by QEMU.

diff --git a/proc.c b/proc.c
index 806b1b1..bd35f7b 100644
--- a/proc.c
+++ b/proc.c
@@ -322,6 +322,7 @@ wait(void)
 void
 scheduler(void)
 {
+  int ran;
   struct proc *p;
   struct cpu *c = mycpu();
   c->proc = 0;
@@ -330,12 +331,16 @@ scheduler(void)
     // Enable interrupts on this processor.
     sti();
 
+    ran = 0;
+
     // Loop over process table looking for process to run.
     acquire(&ptable.lock);
     for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
       if(p->state != RUNNABLE)
         continue;
 
+      ran = 1;
+
       // Switch to chosen process.  It is the process's job
       // to release ptable.lock and then reacquire it
       // before jumping back to us.
@@ -352,6 +357,9 @@ scheduler(void)
     }
     release(&ptable.lock);
 
+    if(ran == 0)
+      hlt();
+
   }
 }
 
diff --git a/x86.h b/x86.h
index 07312a5..578888f 100644
--- a/x86.h
+++ b/x86.h
@@ -144,6 +144,12 @@ lcr3(uint val)
   asm volatile("movl %0,%%cr3" : : "r" (val));
 }
 
+static inline void
+hlt()
+{
+  asm volatile("hlt" : : );
+}
+
 //PAGEBREAK: 36
 // Layout of the trap frame built on the stack by the
 // hardware and by trapasm.S, and passed to trap().