tsc: não é possível calibrar o PIT

Question

$ dmesg | grep tsc
[    0.000000] tsc: Unable to calibrate against PIT
[    0.000000] tsc: using HPET reference calibration
[    0.000000] tsc: Detected 4008.361 MHz processor
[    1.608138] tsc: Refined TSC clocksource calibration: 4008.029 MHz
[    1.608140] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x39c5fcfe866, max_idle_ns: 440795249027 ns
[    2.608431] clocksource: Switched to clocksource tsc

1. O que é um tsc?
2. O que significa Unable to calibrate against PIT ?