x86, apic: APIC code touches invalid MSR on P5 class machines

Current APIC code assumes MSR_IA32_APICBASE is present for all systems.
Pentium Classic P5 and friends didn't have this MSR. MSR_IA32_APICBASE
was introduced as an architectural MSR by Intel @ P6.

Code paths that can touch this MSR invalidly are when vendor == Intel &&
cpu-family == 5 and APIC bit is set in CPUID - or when you simply pass
lapic on the kernel command line, on a P5.

The below patch stops Linux incorrectly interfering with the
MSR_IA32_APICBASE for P5 class machines. Other code paths exist that
touch the MSR - however those paths are not currently reachable for a
conformant P5.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linux.intel.com>
Link: http://lkml.kernel.org/r/4F8EEDD3.1080404@linux.intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org>

arch/x86/kernel/apic/apic.c

@@ -1637,9 +1637,11 @@ static int __init apic_verify(void)
 	mp_lapic_addr = APIC_DEFAULT_PHYS_BASE;
 
 	/* The BIOS may have set up the APIC at some other address */
-	rdmsr(MSR_IA32_APICBASE, l, h);
-	if (l & MSR_IA32_APICBASE_ENABLE)
-		mp_lapic_addr = l & MSR_IA32_APICBASE_BASE;
+	if (boot_cpu_data.x86 >= 6) {
+		rdmsr(MSR_IA32_APICBASE, l, h);
+		if (l & MSR_IA32_APICBASE_ENABLE)
+			mp_lapic_addr = l & MSR_IA32_APICBASE_BASE;
+	}
 
 	pr_info("Found and enabled local APIC!\n");
 	return 0;
@@ -1657,13 +1659,15 @@ int __init apic_force_enable(unsigned long addr)
 	 * MSR. This can only be done in software for Intel P6 or later
 	 * and AMD K7 (Model > 1) or later.
 	 */
-	rdmsr(MSR_IA32_APICBASE, l, h);
-	if (!(l & MSR_IA32_APICBASE_ENABLE)) {
-		pr_info("Local APIC disabled by BIOS -- reenabling.\n");
-		l &= ~MSR_IA32_APICBASE_BASE;
-		l |= MSR_IA32_APICBASE_ENABLE | addr;
-		wrmsr(MSR_IA32_APICBASE, l, h);
-		enabled_via_apicbase = 1;
+	if (boot_cpu_data.x86 >= 6) {
+		rdmsr(MSR_IA32_APICBASE, l, h);
+		if (!(l & MSR_IA32_APICBASE_ENABLE)) {
+			pr_info("Local APIC disabled by BIOS -- reenabling.\n");
+			l &= ~MSR_IA32_APICBASE_BASE;
+			l |= MSR_IA32_APICBASE_ENABLE | addr;
+			wrmsr(MSR_IA32_APICBASE, l, h);
+			enabled_via_apicbase = 1;
+		}
 	}
 	return apic_verify();
 }
@@ -2209,10 +2213,12 @@ static void lapic_resume(void)
 		 * FIXME! This will be wrong if we ever support suspend on
 		 * SMP! We'll need to do this as part of the CPU restore!
 		 */
-		rdmsr(MSR_IA32_APICBASE, l, h);
-		l &= ~MSR_IA32_APICBASE_BASE;
-		l |= MSR_IA32_APICBASE_ENABLE | mp_lapic_addr;
-		wrmsr(MSR_IA32_APICBASE, l, h);
+		if (boot_cpu_data.x86 >= 6) {
+			rdmsr(MSR_IA32_APICBASE, l, h);
+			l &= ~MSR_IA32_APICBASE_BASE;
+			l |= MSR_IA32_APICBASE_ENABLE | mp_lapic_addr;
+			wrmsr(MSR_IA32_APICBASE, l, h);
+		}
 	}
 
 	maxlvt = lapic_get_maxlvt();