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>

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);