APIC IDs may not be consecutive and start from zero, so we cannot really use it

as a direct index into cpus. Record apicid in struct cpu and have cpunum() look for it. Replace cpu->id with cpunum() everywhere, and replace cpu->id with cpu->apicid. Thanks to Xi Wang.

APIC IDs may not be consecutive and start from zero, so we cannot really use it
ae15515d · Frans Kaashoek · 37939f24 · ae15515d · ae15515d · ae15515d
--- a/console.c
+++ b/console.c
@@ -110,7 +110,7 @@ panic(char *s)
  cli();
  cons.locking = 0;
-  cprintf("cpu%d: panic: ", cpu->id);
+  cprintf("cpu with apicid %d: panic: ", cpu->apicid);
  cprintf(s);
  cprintf("\n");
  getcallerpcs(&s, pcs);

--- a/lapic.c
+++ b/lapic.c
 // The local APIC manages internal (non-I/O) interrupts.
 // See Chapter 8 & Appendix C of Intel processor manual volume 3.
+#include "param.h"
 #include "types.h"
 #include "defs.h"
 #include "date.h"
@@ -8,6 +9,7 @@
 #include "traps.h"
 #include "mmu.h"
 #include "x86.h"
+#include "proc.h"  // ncpu
 // Local APIC registers, divided by 4 for use as uint[] indices.
 #define ID      (0x0020/4)   // ID
@@ -99,6 +101,8 @@ lapicinit(void)
 int
 cpunum(void)
 {
+  int apicid, i;
  // Cannot call cpu when interrupts are enabled:
  // result not guaranteed to last long enough to be used!
  // Would prefer to panic but even printing is chancy here:
@@ -111,9 +115,15 @@ cpunum(void)
        __builtin_return_address(0));
  }
-  if(lapic)
+  if (!lapic)
-    return lapic[ID]>>24;
    return 0;
+  apicid = lapic[ID] >> 24;
+  for (i = 0; i < ncpu; ++i) {
+    if (cpus[i].apicid == apicid)
+      return i;
+  }
+  panic("unknown apicid\n");
 }
 // Acknowledge interrupt.

--- a/main.c
+++ b/main.c
@@ -22,7 +22,7 @@ main(void)
  mpinit();        // detect other processors
  lapicinit();     // interrupt controller
  seginit();       // segment descriptors
-  cprintf("\ncpu%d: starting xv6\n\n", cpu->id);
+  cprintf("\ncpu%d: starting xv6\n\n", cpunum());
  picinit();       // another interrupt controller
  ioapicinit();    // another interrupt controller
  consoleinit();   // console hardware
@@ -54,7 +54,7 @@ mpenter(void)
 static void
 mpmain(void)
 {
-  cprintf("cpu%d: starting\n", cpu->id);
+  cprintf("cpu%d: starting\n", cpunum());
  idtinit();       // load idt register
  xchg(&cpu->started, 1); // tell startothers() we're up
  scheduler();     // start running processes
@@ -89,7 +89,7 @@ startothers(void)
    *(void**)(code-8) = mpenter;
    *(int**)(code-12) = (void *) V2P(entrypgdir);
-    lapicstartap(c->id, V2P(code));
+    lapicstartap(c->apicid, V2P(code));
    // wait for cpu to finish mpmain()
    while(c->started == 0)

--- a/mp.c
+++ b/mp.c
@@ -106,12 +106,10 @@ mpinit(void)
    switch(*p){
    case MPPROC:
      proc = (struct mpproc*)p;
-      if(ncpu != proc->apicid){
+      if(ncpu < NCPU) {
-        cprintf("mpinit: ncpu=%d apicid=%d\n", ncpu, proc->apicid);
+        cpus[ncpu].apicid = proc->apicid;  // apicid may differ from ncpu
-        ismp = 0;
-      }
-      cpus[ncpu].id = ncpu;
        ncpu++;
+      }
      p += sizeof(struct mpproc);
      continue;
    case MPIOAPIC:
@@ -125,8 +123,8 @@ mpinit(void)
      p += 8;
      continue;
    default:
-      cprintf("mpinit: unknown config type %x\n", *p);
      ismp = 0;
+      break;
    }
  }
  if(!ismp){

--- a/proc.h
+++ b/proc.h
 // Per-CPU state
 struct cpu {
-  uchar id;                    // Local APIC ID; index into cpus[] below
+  uchar apicid;                // Local APIC ID
  struct context *scheduler;   // swtch() here to enter scheduler
  struct taskstate ts;         // Used by x86 to find stack for interrupt
  struct segdesc gdt[NSEGS];   // x86 global descriptor table

--- a/trap.c
+++ b/trap.c
@@ -48,7 +48,7 @@ trap(struct trapframe *tf)
  switch(tf->trapno){
  case T_IRQ0 + IRQ_TIMER:
-    if(cpu->id == 0){
+    if(cpunum() == 0){
      acquire(&tickslock);
      ticks++;
      wakeup(&ticks);
@@ -74,7 +74,7 @@ trap(struct trapframe *tf)
  case T_IRQ0 + 7:
  case T_IRQ0 + IRQ_SPURIOUS:
    cprintf("cpu%d: spurious interrupt at %x:%x\n",
-            cpu->id, tf->cs, tf->eip);
+            cpunum(), tf->cs, tf->eip);
    lapiceoi();
    break;
@@ -83,13 +83,13 @@ trap(struct trapframe *tf)
    if(proc == 0 || (tf->cs&3) == 0){
      // In kernel, it must be our mistake.
      cprintf("unexpected trap %d from cpu %d eip %x (cr2=0x%x)\n",
-              tf->trapno, cpu->id, tf->eip, rcr2());
+              tf->trapno, cpunum(), tf->eip, rcr2());
      panic("trap");
    }
    // In user space, assume process misbehaved.
    cprintf("pid %d %s: trap %d err %d on cpu %d "
            "eip 0x%x addr 0x%x--kill proc\n",
-            proc->pid, proc->name, tf->trapno, tf->err, cpu->id, tf->eip,
+            proc->pid, proc->name, tf->trapno, tf->err, cpunum(), tf->eip,
            rcr2());
    proc->killed = 1;
  }