Re: kvmclock doesn't work, help?

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On Fri, Dec 18, 2015 at 11:27:13AM -0800, Andy Lutomirski wrote:
> On Fri, Dec 18, 2015 at 3:47 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > On Thu, Dec 17, 2015 at 05:12:59PM -0800, Andy Lutomirski wrote:
> >> On Thu, Dec 17, 2015 at 11:08 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> >> > On Thu, Dec 17, 2015 at 08:33:17AM -0800, Andy Lutomirski wrote:
> >> >> On Wed, Dec 16, 2015 at 1:57 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> >> >> > 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@xxxxxxxxxxxxxx> wrote:
> >> >> >> > On Tue, Dec 15, 2015 at 12:42 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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
> >> >
> >> > (1)
> >> >
> >> >> >> >>> >
> >> >> >> >>> > ...
> >> >> >> >>> >
> >> >> >> >>> > 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).
> >> >>
> >> >> pvclock_clocksource_read?  That seems to do the same thing as all the
> >> >> other clocksource access functions.
> >> >>
> >> >> >
> >> >> > 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).
> >> >>
> >> >> Of course.  But I expected that, in the absence of NTP on the guest,
> >> >> that the guest would track the host's *corrected* 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.
> >> >>
> >> >> Can you point to how this works?  I found kvm_guest_time_update, whch
> >> >> is called under circumstances that I haven't untangled.  I can't
> >> >> really tell what it's trying to do.
> >> >
> >> > Documentation/virtual/kvm/timekeeping.txt.
> >> >
> >>
> >> That document is really long.  I skimmed it and found nothing.
> >
> > kvm_guest_time_update is called when KVM_REQ_UPDATE_CLOCK is set.
> >
> > This happens when:
> >         - kvmclock is enabled or disabled by the guest.
> >         - periodically to propagate NTP correction to kvmclock clock.
> >         - guest vcpu switching between host pcpus when TSCs are out of sync.
> >         - after migration.
> >         - after savevm/loadvm.
> >
> >> >> In any case, this still seems much more convoluted than it has to be.
> >> >> In the case in which the host has a stable TSC (tsc is selected in the
> >> >> core timekeeping code, VCLOCK_TSC is set, etc), which is basically all
> >> >> the time on the last few generations of CPUs, then the core
> >> >> timekeeping code is already exposing a linear function that's supposed
> >> >> to be used for monotonic, cpu-local access to a corrected nanosecond
> >> >> counter.  It's even in pretty much exactly the right form to pass
> >> >> through to the guest via pvclock in the gtod data.  Why doesn't KVM
> >> >> pass it through verbatim, updated in real time?  Is there some legacy
> >> >> reason that KVM must apply its own corrections and has to jump through
> >> >> hoops to pause vcpus when updating those vcpu's copies of the pvclock
> >> >> data?
> >> >
> >> > Read the comment on x86.c which starts with
> >> > " *
> >> >  * Assuming a stable TSC across physical CPUS, and a stable TSC
> >> >  * across virtual CPUs, the following condition is possible.
> >> >  * Each numbered line represents an event visible to both
> >> >  * CPUs at the next numbered event.
> >> > "
> >>
> >> A couple things:
> >>
> >> 1. That says: timespec0 + (rdtsc - tsc0) < timespec0 + N + (rdtsc - (tsc0 + M))
> >>
> >> but that's wrong, I think.  rdtsc is a function, not a number.
> >
> > View it as a number, then its correct.
> >
> >>  Shouldn't it be:
> >>
> >> timespec0 + (rdtsc0 - tsc0) < timespec0 + N + (rdtsc1 - (tsc0 + M))
> >
> > Think "rdtsc" is one number (rdtsc0 = rdtsc1).
> >
> >> which is true iff rdtsc0 < rdtsc1 + N - M, which is equivalent to M <
> >> N + (rdtsc1 - rdtsc0)?
> >>
> >> That doesn't change the conclusion.
> >>
> >> In any case, I'm not arguing that the concept of a master copy is
> >> unnecessary; I'm arguing that the implementation, the calculations,
> >> and the machinations in the code are all very, very complicated.  All
> >> that should be needed is to keep all of the vcpu pvti copies the same
> >> and to make sure that you can't ever have one vcpu see a new copy and
> >> then another vcpu see an old copy.
> >
> > Yes, you can't allow two vcpus to see a different copy of the pvti
> > structure.
> 
> Two options:
> 
> 1. Pause all vcpus, then update all the pvti copies, then unpause all
> vcpus.  This would work, but it's expensive.

This is what is done today.

> 
> 2. Increment all the pvti version numbers, then update all of them,
> then increment the version numbers again.
> 
> I think option 2 is a lot nicer than option 1.

Can you write an actual proposal (with details) that accomodates the
issue described at "Assuming a stable TSC across physical CPUS, and a
stable TSC" ?

Yes it would be nicer, the IPIs (to stop the vcpus) are problematic for
realtime guests.

> >>  You can do that by brute-force
> >> freezing all vcpus on an update (what happens now), or you could do it
> >> by just writing all of the copies at the same time from the same host
> >> cpu *while other vcpus are still running*.
> >
> > Ok, can you do that and guarantee the first copy won't be seen by
> > other vcpus? I don't know how.
> >
> >> For the best outcome, you could offer a pvclock protocol v3 in which
> >> there is literally just one pvti copy shared by all vcpus.
> >
> > Sure, lets write a more formal proposal?
> >
> 
> I can try to sketch something out in the next week or two.  It would
> be basically the same as the current protocol, except that there would
> be a single pvti instead of an array.  In the case where the TSCs are
> out of sync and the host can't synchronize them, then this mechanism
> would return an error and the guest would have to fall back.
> 
> --Andy

I'm fine with a single pvti.

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