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diff --git a/sys/x86/x86/cpu_machdep.c b/sys/x86/x86/cpu_machdep.c
--- a/sys/x86/x86/cpu_machdep.c
+++ b/sys/x86/x86/cpu_machdep.c
@@ -423,7 +423,7 @@
cpu_est_clockrate(int cpu_id, uint64_t *rate)
{
uint64_t tsc1, tsc2;
- uint64_t acnt, mcnt, perf;
+ uint64_t acnt_start, acnt_end, mcnt_start, mcnt_end, perf;
register_t reg;
if (pcpu_find(cpu_id) == NULL || rate == NULL)
@@ -452,15 +452,19 @@
/* Calibrate by measuring a short delay. */
reg = intr_disable();
if (tsc_is_invariant) {
- wrmsr(MSR_MPERF, 0);
- wrmsr(MSR_APERF, 0);
+ mcnt_start = rdmsr(MSR_MPERF);
+ acnt_start = rdmsr(MSR_APERF);
tsc1 = rdtsc();
DELAY(1000);
- mcnt = rdmsr(MSR_MPERF);
- acnt = rdmsr(MSR_APERF);
+ mcnt_end = rdmsr(MSR_MPERF);
+ acnt_end = rdmsr(MSR_APERF);
tsc2 = rdtsc();
intr_restore(reg);
- perf = 1000 * acnt / mcnt;
+ if (mcnt_end == mcnt_start) {
+ tsc_perf_stat = 0;
+ return (EOPNOTSUPP);
+ }
+ perf = 1000 * (acnt_end - acnt_start) / (mcnt_end - mcnt_start);
*rate = (tsc2 - tsc1) * perf;
} else {
tsc1 = rdtsc();
diff --git a/sys/x86/x86/tsc.c b/sys/x86/x86/tsc.c
--- a/sys/x86/x86/tsc.c
+++ b/sys/x86/x86/tsc.c
@@ -433,6 +433,8 @@
void
start_TSC(void)
{
+ uint64_t mperf, aperf;
+
if ((cpu_feature & CPUID_TSC) == 0 || tsc_disabled)
return;
@@ -441,12 +443,13 @@
if (cpu_power_ecx & CPUID_PERF_STAT) {
/*
* XXX Some emulators expose host CPUID without actual support
- * for these MSRs. We must test whether they really work.
+ * for these MSRs, or do work but only for reading. We must test
+ * whether they really work.
*/
- wrmsr(MSR_MPERF, 0);
- wrmsr(MSR_APERF, 0);
+ mperf = rdmsr(MSR_MPERF);
+ aperf = rdmsr(MSR_APERF);
DELAY(10);
- if (rdmsr(MSR_MPERF) > 0 && rdmsr(MSR_APERF) > 0)
+ if (rdmsr(MSR_MPERF) != mperf && rdmsr(MSR_APERF) != aperf
tsc_perf_stat = 1;
}

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