Re: [KVM timekeeping 12/35] Robust TSC compensation

[Zachary Amsden]

On 08/20/2010 07:40 AM, Glauber Costa wrote:
> On Thu, Aug 19, 2010 at 10:07:26PM -1000, Zachary Amsden wrote:
>    
> > Make the match of TSC find TSC writes that are close to each other
> > instead of perfectly identical; this allows the compensator to also
> > work in migration / suspend scenarios.
> >
> > Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/x86.c |   14 ++++++++++----
> >   1 files changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 52680f6..0f3e5fb 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -928,21 +928,27 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> >   	struct kvm *kvm = vcpu->kvm;
> >   	u64 offset, ns, elapsed;
> >   	unsigned long flags;
> > +	s64 sdiff;
> >
> >   	spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
> >   	offset = data - native_read_tsc();
> >   	ns = get_kernel_ns();
> >   	elapsed = ns - kvm->arch.last_tsc_nsec;
> > +	sdiff = data - kvm->arch.last_tsc_write;
> > +	if (sdiff<  0)
> > +		sdiff = -sdiff;
> >
> >   	/*
> > -	 * Special case: identical write to TSC within 5 seconds of
> > +	 * Special case: close write to TSC within 5 seconds of
> >   	 * another CPU is interpreted as an attempt to synchronize
> > -	 * (the 5 seconds is to accomodate host load / swapping).
> > +	 * The 5 seconds is to accomodate host load / swapping as
> > +	 * well as any reset of TSC during the boot process.
> >   	 *
> >   	 * In that case, for a reliable TSC, we can match TSC offsets,
> > -	 * or make a best guest using kernel_ns value.
> > +	 * or make a best guest using elapsed value.
> >   	 */
> > -	if (data == kvm->arch.last_tsc_write&&  elapsed<  5ULL * NSEC_PER_SEC) {
> > +	if (sdiff<  nsec_to_cycles(5ULL * NSEC_PER_SEC)&&
> > +	    elapsed<  5ULL * NSEC_PER_SEC) {
> >   		if (!check_tsc_unstable()) {
> >      
> Isn't 5 way too long for this case?
>
>
>    

It was actually too short for a while, and I didn't realize why until I 
discovered on SVM, the APs were getting the TSC reset after the startup IPI.

In any case, the value is certainly up for debate.  I chose a large 
number because who knows how badly things can get off in the case of 
host overcommit / swapping.

Zach
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