Re: Bug in KVM clock backwards compensation

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-04-28 08:59, Zachary Amsden wrote:
> So I've been going over the new code changes to the TSC related code and
> I don't like one particular set of changes.  In particular, here:
> 
>         kvm_x86_ops->vcpu_load(vcpu, cpu);
>         if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
>                 /* Make sure TSC doesn't go backwards */
>                 s64 tsc_delta;
>                 u64 tsc;
> 
>                 kvm_get_msr(vcpu, MSR_IA32_TSC, &tsc);
>                 tsc_delta = !vcpu->arch.last_guest_tsc ? 0 :
>                              tsc - vcpu->arch.last_guest_tsc;
> 
>                 if (tsc_delta < 0)
>                         mark_tsc_unstable("KVM discovered backwards TSC");
>                 if (check_tsc_unstable()) {
>                         kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
>                         vcpu->arch.tsc_catchup = 1;
>                 }
> 
> 
> The point of this code fragment is to test the host clock to see if it
> is stable, because we may have just come back from an idle phase which
> stopped the TSC, switched CPUs, or come back from a deep sleep state
> which reset the host TSC.
> 
> However, the above code is fetching the guest TSC instead of the host
> TSC, which isn't the way it is supposed to work.
> 
> I saw a patch floating around that touched this code recently, but I
> think there's a definite issue here that needs addressing.

And /me still wonders (like I did when this first popped up) if the
proper place of determining TSC stability really have to be KVM.

If the Linux core fails to detect some instability and KVM has to jump
in, shouldn't we better improve the core's detection abilities and make
use of them in KVM? Conceptually this looks like we are currently just
working around a core deficit in KVM.

Jan


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