From e94d5ad227abd9acd71bf4ca8230530017833a14 Mon Sep 17 00:00:00 2001
From: Jung-uk Kim <jkim@FreeBSD.org>
Date: Tue, 12 Apr 2011 22:41:52 +0000
Subject: [PATCH] Do not use TSC for DELAY(9) if it not P-state invariant to
 avoid possible foot-shooting.  DELAY() becomes unreliable when TSC frequency
 varies wildly, especially cpufreq(4) and powerd(8) are used at the same time.

---
 sys/x86/isa/clock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sys/x86/isa/clock.c b/sys/x86/isa/clock.c
index d82dad7f4948..5556ee651a7f 100644
--- a/sys/x86/isa/clock.c
+++ b/sys/x86/isa/clock.c
@@ -263,7 +263,7 @@ delay_tc(int n)
 
 	tc = timecounter;
 	freq = atomic_load_acq_64(&tsc_freq);
-	if (freq != 0) {
+	if (tsc_is_invariant && freq != 0) {
 		func = get_tsc;
 		mask = ~0u;
 	} else {