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    SubjectRe: [patch 4/9] Remove the TSC synchronization on SMP machines
    Andi Kleen writes:

    > Just to avoid spreading misinformation: modulo some new broken hardware
    > (which we always try to work around when found) i386/x86-64 gettimeofday
    > is monotonic. AFAIK on the currently known hardware it should be generally
    > ok.
    >
    > However ntpd can always screw you up, but that's inherent in the design.

    On powerpc we manage to keep gettimeofday monotonic even when ntpd is
    adjusting the clock. We have 3 parameters used to convert a value
    from the timebase register to the time of day, and these parameters
    are adjusted if necessary at the beginning of each tick, based on the
    value returned by current_tick_length(). The point is that
    current_tick_length() tells you at the *beginning* of each tick how
    much time will be added on to xtime at the *end* of that tick, and
    that makes it possible to aim the interpolation to hit the same value
    as xtime at the end of each tick.

    Clearly if you make a discrete jump backwards with settimeofday or
    adjtime, it's impossible to keep gettimeofday monotonic, but apart
    from that it's monotonic on powerpc.

    At least, that's the way it's supposed to work. I hope the recent
    timekeeping changes haven't broken it. :)

    Paul.
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