Re: [PATCH v3 2/6] KVM: x86: switch to masterclock update using timekeeper functionality

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/08/2017 14:11, Denis Plotnikov wrote:
> In fact, this "cycles_valid" is going to be used for deciding whether to
> use KVM masterclock or not. And if it's not we still want to know
> cycles_stamp value to use it in KVM.

Why?  Neither pvclock_update_vm_gtod_copy nor kvm_pv_clock_pairing do
anything with the two variables that are passed by reference, if the
read returns false.  Hence my suggestion of calling it cycles_valid.

> So the cycles is valid, but clocksource is not reliable (why? let decide
> to a clocksource, by default assume they are all not stable), thus we
> must calculate time values for a guest each time its needed.
> So, my proposal is to name the variable sightly differently: cs_reliable
> and go like:
>     if (clock->read_clock_with_stamp) {
>         systime_snapshot->cs_reliable =
>             clock->read_clock_with_stamp(
>                 &now, &systime_snapshot->cycles);
>     } else {
>         now = tk_clock_read(&tk->tkr_mono);
>         systime_snapshot->cs_reliable = false;
>         systime_snapshot->cycles = now;
>     }
> What do you think?

I'm afraid you still have to define the meaning of "reliable".  (Though
I agree that the right default is false, that was a thinko on my side.
This also means that you need to define read_clock_with_stamp for the
TSC clocksource too).

Paolo