On Fri, Jan 20, 2017 at 07:08:49PM +0100, Radim Krcmar wrote:
> 2017-01-20 13:00-0200, Marcelo Tosatti:
> > On Fri, Jan 20, 2017 at 03:12:56PM +0100, Radim Krcmar wrote:
> >> 2017-01-20 10:20-0200, Marcelo Tosatti:
> >> > + do {
> >> > + /*
> >> > + * We are measuring the delay between
> >> > + * kvm_hypercall and rdtsc using TSC,
> >> > + * and converting that delta to
> >> > + * tsc_to_system_mul and tsc_shift
> >> > + * So any changes to tsc_to_system_mul
> >> > + * and tsc_shift in this region
> >> > + * invalidate the measurement.
> >> > + */
> >> > + version = pvclock_read_begin(src);
> >> > +
> >> > + ret = kvm_hypercall2(KVM_HC_CLOCK_PAIRING,
> >> > + clock_off_gpa,
> >> > + KVM_CLOCK_PAIRING_WALLCLOCK);
> >> > + if (ret != 0) {
> >> > + pr_err("clock offset hypercall ret %lu\n", ret);
> >> > + spin_unlock(&kvm_ptp_lock);
> >> > + preempt_enable_notrace();
> >> > + return -EOPNOTSUPP;
> >> > + }
> >> > +
> >> > + tspec.tv_sec = clock_off.sec;
> >> > + tspec.tv_nsec = clock_off.nsec;
> >> > + ret = __pvclock_read_cycles(src, clock_off.tsc);
> >> > + flags = src->flags;
> >> > + } while (pvclock_read_retry(src, version));
> >> > +
> >> > + preempt_enable_notrace();
> >> > +
> >> > + system_counter->cycles = ret;
> >> > + system_counter->cs = get_kvmclock_cs();
> >>
> >> Can't we use clocksource_tsc and just pass the tsc without kvmclock in
> >> the middle?
> >
> > No, it has to be the kvmclock value.
>
> What happens if the guest switches from kvmclock to tsc?
The ioctl will return -ENODEV.
>From get_device_system_crosststamp function:
/*
* Verify that the clocksource associated with the
* captured
* system counter value is the same as the currently
* installed
* timekeeper clocksource
*/
if (tk->tkr_mono.clock != system_counterval.cs)
return -ENODEV;
