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    SubjectRe: [3/8] x86/tsc: Store and check TSC ADJUST MSR
    Henning,

    On Fri, 27 Jan 2017, Henning Schild wrote:
    >
    > did you by any chance look into TSC synchronization by adjusting the
    > absolute value (MSR_IA32_TSC) as well? As far as i have seen Linux did
    > that a long time ago and eventually it was stopped because it caused more
    > harm than good.

    I was involved in both developing the TSC sync patches and ripping them out
    again.

    The problem with writing TSC directly is that you really _CANNOT_ reliably
    handle run-time differences and SMI/NMI induced deltas. With the adjust MRS
    it's a halfways sane thing to do, except for the brokeness of that botch
    job vs. the TSC deadline timer.

    > The ADJUST MSR offers an easy way to synchronize, still taking care of
    > all the special cases resulted in an 8-patch series. Synching without
    > that using the absolute value is likely much harder, but that series
    > might be a good foundation.

    I'm not even thinking about bringing the pure TSC based sync back.

    > The big question is whether we can rely on all future CPUs to
    > support that MSR. Do "new MSRs" disappear again at some point? If we
    > can not rely on the ADJUST MSR, now might be a good time to revisit the
    > idea of synching the absolute values.

    There is nothing you can ever be sure about, but I doubt that the ADJUST
    MSR is going to vanish.

    > I remember having read somewhere that this series might get backported
    > to longterm kernels, what is the status on that?

    No idea.

    Thanks,

    tglx

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