Partition Reference Time Enlightenment

The partition reference time enlightenment presents a reference time source that does not require an intercept into the hypervisor. This enlightenment is available only when the underlying platform provides support of an invariant processor Time Stamp Counter (TSC), or iTSC. In such platforms, the processor TSC frequency remains constant irrespective of changes in the processor's clock frequency. The processor's clock frequency might change from the use of power management states such as ACPI processor performance states, processor idle sleep states (ACPI C-states), and so on.

The partition reference time enlightenment uses a virtual TSC value, an offset, and a multiplier to enable a guest partition to compute the normalized reference time since partition creation, in 100 nanosecond units. The partition reference time enlightenment also allows a guest partition to atomically compute the reference time when the guest partition is migrated to a platform with a different TSC rate. The partition reference time enlightenment provides a fall-back mechanism to support migration to platforms without the constant rate TSC feature.

This facility is not intended to be used as a source of wall clock time because the reference time that is computed by using this facility appears to stop when a guest partition is saved until the subsequent restore.

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Build date: 11/16/2013