mirage-clock -- libraries and module types for portable clocks

This library implements portable support for an operating system timesource
that is compatible with the MirageOS library interfaces
found in: https://github.com/mirage/mirage

It implements an MCLOCK module that represents a monotonic timesource
since an arbitrary point, and PCLOCK which counts time since the Unix
epoch.

The following sources are used:

• The Unix version uses gettimeofday or clock_gettime, depending on
  which OS is in use (see clock_stubs.c).

• The freestanding version uses the paravirtual clock source from the hypervisor.