Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

[Avi Kivity]

 On 09/14/2010 11:10 AM, Jan Kiszka wrote:
> Am 20.08.2010 10:07, Zachary Amsden wrote:
> > When CPUs with unstable TSCs enter deep C-state, TSC may stop
> > running.  This causes us to require resynchronization.  Since
> > we can't tell when this may potentially happen, we assume the
> > worst by forcing re-compensation for it at every point the VCPU
> > task is descheduled.
> >
> > Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/x86.c |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 7fc4a55..52b6c21 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1866,7 +1866,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >   	}
> >
> >   	kvm_x86_ops->vcpu_load(vcpu, cpu);
> > -	if (unlikely(vcpu->cpu != cpu)) {
> > +	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
> >   		/* Make sure TSC doesn't go backwards */
> >   		s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
> >   				native_read_tsc() - vcpu->arch.last_host_tsc;
> For yet unknown reason, this commit breaks Linux guests here if they are
> started with only a single VCPU. They hang during boot, obviously no
> longer receiving interrupts.
>
> I'm using kvm-kmod against a 2.6.34 host kernel, so this may be a side
> effect of the wrapping, though I cannot imagine how.
>
> Anyone any ideas?

Most likely, time went backwards, and some 'future - past' calculation 
resulted in a negative sleep value which was then interpreted as 
unsigned and resulted in a 2342525634 year sleep.

Does your guest use kvmclock, tsc, or some other time source?

--
error compiling committee.c: too many arguments to function

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