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', e.g.,
a 2.5GHz CPU can be detected as 1.3GHz.
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 should use Hyper-V time counter, which is much more reliable than i8254,
to calibrate TSC.
And, we must use Hyper-V timer counter for Hyper-V Generation-2 (UEFI) VM,
because i8254 doesn't exist for such a VM.
Please suggest how I should improve the patch, e.g., should I move
the new hyperv_delay() to a new file?
Submitted by: Dexuan Cui <decui@microsoft.com>