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>

1 files changed, 20 insertions, 14 deletions

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 11544d8f1e97..edc24480469f 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/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();