From 0e78005e5ca1323a9c4eefca6f77157101d9e53b Mon Sep 17 00:00:00 2001
From: Jung-uk Kim <jkim@FreeBSD.org>
Date: Thu, 14 Apr 2011 17:50:26 +0000
Subject: [PATCH] Work around an emulator problem where virtual CPU advertises
 TSC is P-state invariant and APERF/MPERF MSRs exist but these MSRs never
 tick.  When we calculate effective frequency from cpu_est_clockrate(), it
 caused panic of division-by-zero.  Now we test whether these MSRs actually
 increase to avoid such foot-shooting.

Reported by:	dim
Tested by:	dim
---
 sys/x86/x86/tsc.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/sys/x86/x86/tsc.c b/sys/x86/x86/tsc.c
index 04e936a535a2..c08f4adc8e62 100644
--- a/sys/x86/x86/tsc.c
+++ b/sys/x86/x86/tsc.c
@@ -183,8 +183,18 @@ probe_tsc_freq(void)
 
 	if (cpu_high >= 6) {
 		do_cpuid(6, regs);
-		if ((regs[2] & CPUID_PERF_STAT) != 0)
-			tsc_perf_stat = 1;
+		if ((regs[2] & CPUID_PERF_STAT) != 0) {
+			/*
+			 * XXX Some emulators expose host CPUID without actual
+			 * support for these MSRs.  We must test whether they
+			 * really work.
+			 */
+			wrmsr(MSR_MPERF, 0);
+			wrmsr(MSR_APERF, 0);
+			DELAY(10);
+			if (rdmsr(MSR_MPERF) > 0 && rdmsr(MSR_APERF) > 0)
+				tsc_perf_stat = 1;
+		}
 	}
 
 	if (tsc_skip_calibration) {