On 06/27/2018 13:01, Ryan Stone wrote: > 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.
I think it was legacy USB device emulation for certain Intel chipset-based motherboards. Jung-uk Kim
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