As part of an investigation into NTP time synchronization issues under bhyve, we found that use of sbintime_t over struct bintime for the vPIT emulation resulted in an unnecessary loss of precision that negatively impacted consumers' ability to use it as an accurate time reference. Ignoring timer/callout implementation details, the lack of precision itself was causing a 59ppm error, a not-so-insignificant fraction of the 500ppm divergence budget which some NTP implementations possess.

The full write-up can be found in OS-6923

During development of the change, APIC timer debugging information from a Linux guest (compare against the expected 838860 ticks) was used to judge performance of the emulated PIT.

Stock:

838753 - 107    (128 ppm)
838776 - 84     (100 ppm)
838774 - 86     (103 ppm)
838706 - 154    (184 ppm)
838768 - 92     (110 ppm)
838769 - 91     (108 ppm)

Patched:

838922 - 62     (74 ppm)
838903 - 43     (51 ppm)
838908 - 48     (57 ppm)
838906 - 46     (55 ppm)
838905 - 45     (54 ppm)
838907 - 47     (56 ppm)

This change has been in SmartOS bhyve for almost a year.