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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>
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