TSC calibration in virtual machines

[Ryan Stone]

I would guess that the calibration can fail because when running under
the hypervisor, the FreeBSD guest code can be descheduled at the wrong
time.  As I recall, the current algorithm looks like:

1. Sample rdtsc
2. Use a fixed-frequency timer to busy-wait for exactly 1 second
3. Sample rdtsc again
4. tsc_freq = sample2 - sample1;

If we are descheduled between 2 and 3, the time we spend off-cpu will
not be accounted for at step 4.  On bare-metal this is not possible as
neither the scheduler nor interrupts are not running yet.

Although, come to think of it, I seem to recall something about SMI
interrupts mucking this up long in the past, for exactly the same
reason.