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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
@@ -240,7 +240,7 @@
}
static void
-tsc_freq_8254(uint64_t *res)
+tsc_freq_tc(uint64_t *res)
{
uint64_t tsc1, tsc2;
int64_t overhead;
@@ -262,8 +262,15 @@
tsc_freq = (tsc2 - tsc1 - overhead) * 10;
}
+/*
+ * Try to determine the TSC frequency using CPUID or hypercalls. If successful,
+ * this lets use the TSC for early DELAY() calls instead of the 8254 timer,
+ * which may be unreliable or entirely absent on contemporary systems. However,
+ * avoid calibrating using the 8254 here so as to give hypervisors a chance to
+ * register a timecounter that can be used instead.
+ */
static void
-probe_tsc_freq(void)
+probe_tsc_freq_early(void)
{
#ifdef __i386__
/* The TSC is known to be broken on certain CPUs. */
@@ -364,7 +371,20 @@
if (bootverbose)
printf("Early TSC frequency %juHz derived from CPUID\n",
(uintmax_t)tsc_freq);
- } else if (tsc_skip_calibration) {
+ }
+}
+
+/*
+ * If we were unable to determine the TSC frequency via CPU registers, try
+ * to calibrate against a known clock.
+ */
+static void
+probe_tsc_freq_late(void)
+{
+ if (tsc_freq != 0)
+ return;
+
+ if (tsc_skip_calibration) {
/*
* Try to parse the brand string to obtain the nominal TSC
* frequency.
@@ -380,15 +400,24 @@
}
} else {
/*
- * Calibrate against the 8254 PIT. This estimate will be
- * refined later in tsc_calib().
+ * Calibrate against a timecounter or the 8254 PIT. This
+ * estimate will be refined later in tsc_calib().
*/
- tsc_freq_8254(&tsc_freq);
+ tsc_freq_tc(&tsc_freq);
if (bootverbose)
printf(
"Early TSC frequency %juHz calibrated from 8254 PIT\n",
(uintmax_t)tsc_freq);
}
+}
+
+void
+start_TSC(void)
+{
+ if ((cpu_feature & CPUID_TSC) == 0 || tsc_disabled)
+ return;
+
+ probe_tsc_freq_late();
if (cpu_power_ecx & CPUID_PERF_STAT) {
/*
@@ -401,13 +430,6 @@
if (rdmsr(MSR_MPERF) > 0 && rdmsr(MSR_APERF) > 0)
tsc_perf_stat = 1;
}
-}
-
-void
-start_TSC(void)
-{
- if ((cpu_feature & CPUID_TSC) == 0 || tsc_disabled)
- return;
/*
* Inform CPU accounting about our boot-time clock rate. This will
@@ -718,7 +740,7 @@
if ((cpu_feature & CPUID_TSC) == 0 || tsc_disabled)
return;
- probe_tsc_freq();
+ probe_tsc_freq_early();
}
/*

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