x86: warn of incorrect cpu_khz on AMD systems

Impact: add debug check

If none of the perfctrs are free when calculating cpu_khz we default to
using ctr 3 (ie, we just choose 3).  This may lead to an incorrect tsc
freq value which can cause the system to be unstable.

To aid in future debugging, WARN the user of a potential problem.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/x86/kernel/time_64.c b/arch/x86/kernel/time_64.c
index cb19d650c216..418a095c5796 100644
--- a/arch/x86/kernel/time_64.c
+++ b/arch/x86/kernel/time_64.c
@@ -80,6 +80,8 @@ unsigned long __init calibrate_cpu(void)
                         break;
         no_ctr_free = (i == 4);
         if (no_ctr_free) {
+                WARN(1, KERN_WARNING "Warning: AMD perfctrs busy ... "
+                     "cpu_khz value may be incorrect.\n");
                 i = 3;
                 rdmsrl(MSR_K7_EVNTSEL3, evntsel3);
                 wrmsrl(MSR_K7_EVNTSEL3, 0);