On Wed, Sep 16, 2020 at 01:59:43PM +0200, Vitaly Kuznetsov wrote: > "Dr. David Alan Gilbert" <dgilb...@redhat.com> writes: > > > cc'ing in Vitaly who knows about the hv stuff. > > > > cc'ing Marcelo who knows about clocksources :-) > > > * Antoine Damhet (antoine.dam...@blade-group.com) wrote: > >> Hi, > >> > >> We are experiencing timestamp rollbacks during live-migration of > >> Windows 10 guests > > Are you migrating to the same hardware (with the same TSC frequency)? Is > TSC used as the clocksource on the host?
Yes we are migrating to the exact same hardware. And yes TSC is used as
a clocksource in the host (but the bug is still happening with `hpet` as
a clocksource).
>
> >> with the following qemu configuration (linux 5.4.46
> >> and qemu master):
> >> ```
> >> $ qemu-system-x86_64 -enable-kvm -cpu host,kvm=off,hv_time [...]
> >> ```
>
> Out of pure curiosity, what's the purpose of doing 'kvm=off'? Windows is
> not going to check for KVM identification anyway so we pretend we're
> Hyper-V.
Some softwares explicitly checks for the presence of KVM and then crash
if they find it in CPUID :/
>
> Also, have you tried adding more Hyper-V enlightenments?
Yes, I published a stripped-down command-line for a minimal reproducer
but even `hv-frequencies` and `hv-reenlightenment` don't help.
>
> >
> > How big a jump are you seeing, and how did you notice it in the guest?
> >
> > Dave
> >
> >> I have tracked the bug to the fact that `kvmclock` is not exposed and
> >> disabled from qemu PoV but is in fact used by `hv-time` (in KVM).
> >>
> >> I think we should enable the `kvmclock` (qemu device) if `hv-time` is
> >> present and add Hyper-V support for the `kvmclock_current_nsec`
> >> function.
>
> AFAICT kvmclock_current_nsec() checks whether kvmclock was enabled by
> the guest:
>
> if (!(env->system_time_msr & 1ULL)) {
> /* KVM clock not active */
> return 0;
> }
>
> and this is (and way) always false for Windows guests.
Hooo, I missed this piece. When is `clock_is_reliable` expected to be
false ? Because if it is I still think we should be able to query at
least `HV_X64_MSR_REFERENCE_TSC`
>
> >>
> >> I'm asking for advice because I am unsure this is the _right_ approach
> >> and how to keep migration compatibility between qemu versions.
> >>
> >> Thank you all,
> >>
> >> --
> >> Antoine 'xdbob' Damhet
>
> --
> Vitaly
>
--
Antoine 'xdbob' Damhet
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