Philippe Gerum authored Jan 23, 2021 and Jan Kiszka committed Mar 08, 2021

Dovetail enables out-of-band access to the vDSO-based clock_gettime()
vcall from applications. If present, select this method instead of
relying on the hardware tick counter for CLOCK_MONOTONIC,
CLOCK_MONOTONIC_RAW, CLOCK_REALTIME and CLOCK_HOST_REALTIME.

At binding time, receiving a null hardware clock frequency from the
core means that we should obtain timestamps directly from the
vDSO-based clock_gettime() vcall (see cobalt_use_legacy_tsc()).

In this mode, Cobalt shares the in-band kernel's idea of time for all
common clocks such as CLOCK_MONOTONIC* and CLOCK_REALTIME. As a
result, CLOCK_HOST_REALTIME refers to the common CLOCK_REALTIME clock.
Furthermore, libcobalt's clock_settime(CLOCK_REALTIME) is delegated to
the underlying *libc, which means the caller may switch to secondary
mode.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>