FIXME: use hyper-v time counter (rather than i8254) to calibrate TSC

The i8254 PIT counter emulated by Hyper-V is not reliable.

In probe_tsc_freq() -> DELAY -> init_ops.early_delay() -> i8254_delay(),
getit() can return these values when it is invoked 10 times:
  1 pit count0 = 228
  2 pit count0 = 131
  3 pit count0 = 34
  4 pit count0 = 65473 <-- this is a normal wrap-around.
  5 pit count0 = 65375
  6 pit count0 = 65278
  7 pit count0 = 65180
  8 pit count0 = 65388 <-- this is bad!
  9 pit count0 = 65290
 10 pit count0 = 65193

For the 8th time, the 'delta' in i8254_delay() is < 0 while
it shouldn't, so the later "delta += i8254_max_count;" and
"ticks_left -= delta" will cause i8254_delay() to wait shorter than
expected, and finally probe_tsc_freq() get a smaller 'tsc_freq'.

A smaller 'tsc_freq' can cause time inaccuracy in dtrace.
It can also cause warnings like

calcru: runtime went backwards from 50 usec to 25 usec for pid 0 (kernel)
calcru: runtime went backwards from 1471 usec to 743 usec for pid 0 (kernel)
calcru: runtime went backwards from 40 usec to 20 usec for pid 0 (kernel)
calcru: runtime went backwards from 18 usec to 9 usec for pid 0 (kernel)
calcru: runtime went backwards from 46204978 usec to 23362331 usec for pid 0 (kernel)

We use Hyper-V time counter, which is much more reliable than i8254,
to calibrate TSC.

Signed-off-by: Dexuan Cui <decui@microsoft.com>

Author: dcui

sys/x86/x86/delay.c

@@ -95,12 +95,23 @@ delay_tc(int n)
 	return (1);
 }
 
+#define HV_X64_MSR_TIME_REF_COUNT	0x40000020
 void
 DELAY(int n)
 {
+	uint64_t now, end;
 
 	if (delay_tc(n))
 		return;
+//FIXME: XXX: need to find a way to do this cleanly...
+#if 1
+	now = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
+	end = now + n * 10;
+
+	while (now < end)
+		now = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
+#else
 
 	init_ops.early_delay(n);
+#endif
 }