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    SubjectRe: [PATCH 4/4] KVM: x86: track actual TSC frequency from the timekeeper struct
    On Tue, Feb 16, 2016 at 02:48:16PM +0100, Marcelo Tosatti wrote:
    > On Mon, Feb 08, 2016 at 04:18:31PM +0100, Paolo Bonzini wrote:
    > > When an NTP server is running, it may adjust the time substantially
    > > compared to the "official" frequency of the TSC. A 12 ppm change
    > > sums up to one second per day.
    > >
    > > This already shows up if the guest compares kvmclock with e.g. the
    > > PM timer. It shows up even more once we add support for the Hyper-V
    > > TSC page, because the guest expects it to be in sync with the time
    > > reference counter; effectively the time reference counter is just a
    > > slow path to access the same clock that is in the TSC page.
    > >
    > > Therefore, we want kvmclock to provide the host kernel's
    > > ktime_get_boot_ns() value, at least if the master clock is active.
    > > To do so, reverse-compute the host's "actual" TSC frequency from
    > > pvclock_gtod_data and return it from kvm_get_time_and_clockread.
    >
    > Paolo,
    >
    > You'd have to generate an update to the guest structures as well,
    > to reflect the new {mult,shift} values calculated by the host.
    > Here:
    >
    > /* disable master clock if host does not trust, or does not
    > * use, TSC clocksource
    > */
    > if (gtod->clock.vclock_mode != VCLOCK_TSC &&
    > atomic_read(&kvm_guest_has_master_clock) != 0)
    > queue_work(system_long_wq, &pvclock_gtod_work);
    >
    > No?
    >
    > At first, i'm afraid this might be heavy, so it might be interesting
    > to rate limit the update operation.
    >

    Paolo,

    I suppose its not sufficient:

    500ppm of 300 seconds = .0005*300 = 0.15 seconds.

    Should aim at avoiding time backwards event in the following situation:


    T1) t1_kvmclock_read = get_nanoseconds();
    /* NTP correction to kernel clock = 500ppm */
    /* TSC correction via mult,shift = 0ppm */

    VM-exit, update kvmclock (or Hyper-V) clock data with
    new values

    T2) t2_kvmclock_read = get_nanoseconds();
    /* NTP correction to kernel clock = 500ppm */
    /* TSC correction via mult,shift = 500ppm */


    So should not allow the host clock (or system_timestamp) to diverge
    from (TSC based calculation) more than the duration of the event:

    VM-exit, update kvmclock (or Hyper-V) with new data.

    To avoid t2_kvmclock_read < t1_kvmclock_read

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