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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>
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