A tiny bit of clean up (e.g., move code searching cpu array from lapic.c into

mycpu() in proc.c.

proc.c

@@ -32,13 +32,24 @@ cpuid() {
   return mycpu()-cpus;
 }
 
-// Must be called with interrupts disabled
+// Must be called with interrupts disabled to avoid the caller being rescheduled
+// between reading lapicid and running through the loop.
 struct cpu*
 mycpu(void)
 {
+  int apicid, i;
+  
   if(readeflags()&FL_IF)
     panic("mycpu called with interrupts enabled\n");
-  return &cpus[lapiccpunum()];
+  
+  apicid = lapicid();
+  // APIC IDs are not guaranteed to be contiguous. Maybe we should have
+  // a reverse map, or reserve a register to store &cpus[i].
+  for (i = 0; i < ncpu; ++i) {
+    if (cpus[i].apicid == apicid)
+      return &cpus[i];
+  }
+  panic("unknown apicid\n");
 }
 
 // Disable interrupts so that we are not rescheduled