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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
