Re: kvmclock doesn't work, help?

Marcelo Tosatti
2015-12-17T15:57:39Z

On Wed, Dec 16, 2015 at 10:17:16AM -0800, Andy Lutomirski wrote:
> On Wed, Dec 16, 2015 at 9:48 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Tue, Dec 15, 2015 at 12:42 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>
> >>
> >> On 14/12/2015 23:31, Andy Lutomirski wrote:
> >>> > RAW TSC NTP corrected TSC
> >>> > t0 10 10
> >>> > t1 20 19.99
> >>> > t2 30 29.98
> >>> > t3 40 39.97
> >>> > t4 50 49.96
> >>> >
> >>> > ...
> >>> >
> >>> > if you suddenly switch from RAW TSC to NTP corrected TSC,
> >>> > you can see what will happen.
> >>>
> >>> Sure, but why would you ever switch from one to the other?
> >>
> >> The guest uses the raw TSC and systemtime = 0 until suspend. After
> >> resume, the TSC certainly increases at the same rate as before, but the
> >> raw TSC restarted counting from 0 and systemtime has increased slower
> >> than the guest kvmclock.
> >
> > Wait, are we talking about the host's NTP or the guest's NTP?
> >
> > If it's the host's, then wouldn't systemtime be reset after resume to
> > the NTP corrected value? If so, the guest wouldn't see time go
> > backwards.
> >
> > If it's the guest's, then the guest's NTP correction is applied on top
> > of kvmclock, and this shouldn't matter.
> >
> > I still feel like I'm missing something very basic here.
> >
>
> OK, I think I get it.
>
> Marcelo, I thought that kvmclock was supposed to propagate the host's
> correction to the guest. If it did, indeed, propagate the correction
> then, after resume, the host's new system_time would match the guest's
> idea of it (after accounting for the guest's long nap), and I don't
> think there would be a problem.
> That being said, I can't find the code in the masterclock stuff that
> would actually do this.

Guest clock is maintained by guest timekeeping code, which does:

timer_interrupt()
offset = read clocksource since last timer interrupt
accumulate_to_systemclock(offset)

The frequency correction of NTP in the host can be applied to
kvmclock, which will be visible to the guest
at "read clocksource since last timer interrupt"
(kvmclock_clocksource_read function).

This does not mean that the NTP correction in the host is propagated
to the guests system clock directly.

(For example, the guest can run NTP which is free to do further
adjustments at "accumulate_to_systemclock(offset)" time).

> If, on the other hand, the host's NTP correction is not supposed to
> propagate to the guest,

This is optional. There is a module option to control this, in fact.

Its nice to have, because then you can execute a guest without NTP
(say without network connection), and have a kvmclock (kvmclock is a
clocksource, not a guest system clock) which is NTP corrected.

> then shouldn't KVM just update system_time on
> resume to whatever the guest would think it had (which I think would
> be equivalent to the host's CLOCK_MONOTONIC_RAW value, possibly
> shifted by some per-guest constant offset).
>
> --Andy

Sure, you could add a correction to compensate and make sure
the guest clock does not see time backwards.