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
