On Tue, Oct 03, 2023, David Woodhouse wrote: > > > On 3 October 2023 01:53:11 BST, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > >I think there is still use for synchronizing with the host's view of time, e.g. > >to deal with lost time across host suspend+resume. > > > >So I don't think we can completely sever KVM's paravirt clocks from host time, > >at least not without harming use cases that rely on the host's view to keep > >accurate time. And honestly at that point, the right answer would be to stop > >advertising paravirt clocks entirely. > > > >But I do think we can address the issues that Dongli and David are obversing > >where guest time drifts even though the host kernel's base time hasn't changed. > >If I've pieced everything together correctly, the drift can be eliminated simply > >by using the paravirt clock algorithm when converting the delta from the raw TSC > >to nanoseconds. > > > >This is *very* lightly tested, as in it compiles and doesn't explode, but that's > >about all I've tested. > > Hm, I don't think I like this. Yeah, I don't like it either. I'll respond to your other mail with details, but this is a dead end anything. > You're making get_monotonic_raw() not *actually* return the monotonic_raw > clock, but basically return the kvmclock instead? And why? So that when KVM > attempts to synchronize the kvmclock to the monotonic_raw clock, it gets > tricked into actually synchronizing the kvmclock to *itself*? > > If you get this right, don't we have a fairly complex piece of code that has > precisely *no* effect? > > Can't we just *refrain* from synchronizing the kvmclock to *anything*, in the > CONSTANT_TSC case? Why do we do that anyway? > > (Suspend/resume, live update and live migration are different. In *those* > cases we may need to preserve both the guest TSC and kvmclock based on either > the host TSC or CLOCK_TAI. But that's different.) The issue is that the timekeeping code doesn't provide a notification mechanism to *just* get updates for things like suspend/reume. We could maybe do something in KVM like unregister the notifier if the TSC is constant, and manually refresh on suspend/resume. But that's pretty gross too, and I'd definitely be concerned that we missed something.
