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

[Zachary Amsden]

On 09/14/2010 12:40 AM, Jan Kiszka wrote:
> Am 14.09.2010 11:27, Avi Kivity wrote:
>    
> >    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.
> >      
> Looks like that's the case on first glance at the apic state.
>    

This compensation effectively nulls the delta between current and last TSC:

        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;
                if (tsc_delta < 0)
                        mark_tsc_unstable("KVM discovered backwards TSC");
                if (check_tsc_unstable())
                        kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
                kvm_migrate_timers(vcpu);
                vcpu->cpu = cpu;

If TSC has advanced quite a bit due to a TSC jump during sleep(*), it 
will adjust the offset backwards to compensate; similarly, if it has 
gone backwards, it will advance the offset.

In neither case should the visible TSC go backwards, assuming 
last_host_tsc is recorded properly, and so kvmclock should be similarly 
unaffected.

Perhaps the guest is more intelligent than we hope, and is comparing two 
different clocks: kvmclock or TSC with the rate of PIT interrupts.  This 
could result in negative arithmetic begin interpreted as unsigned.  Are 
you using PIT interrupt reinjection on this guest or passing 
-no-kvm-pit-reinjection?

>    
> > Does your guest use kvmclock, tsc, or some other time source?
> >      
> A kernel that has kvmclock support even hangs in SMP mode. The others
> pick hpet or acpi_pm. TSC is considered unstable.
>    

SMP mode here has always and will always be unreliable.  Are you running 
on an Intel or AMD CPU?  The origin of this code comes from a workaround 
for (*) in vendor-specific code, and perhaps it is inappropriate for both.

Zach
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