Re: [RFC PATCH V2 4/4] Utilize the vmx preemption timer for tsc deadline timer

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/05/2016 15:27, Radim Krčmář wrote:
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > @@ -1343,6 +1343,85 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
> > +void kvm_lapic_arm_hv_timer(struct kvm_vcpu *vcpu)
> > +{
> 
> Preemption timer needs to be adjusted on every vmentry while hrtimer is
> set only once.  After how many vmentries does preemption timer surpass
> the overhead of hrtimer?

See my review. :)  I hope to get the per-vmentry cost of the preemption
timer down to a dozen instructions.

> > +	struct kvm_lapic *apic = vcpu->arch.apic;
> > +	u64 tscdeadline, guest_tsc;
> > +
> > +	if (apic->lapic_timer.hv_timer_state == HV_TIMER_NOT_USED)
> > +		return;
> > +
> > +	tscdeadline = apic->lapic_timer.tscdeadline;
> > +	guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> > +
> > +	if (tscdeadline >= guest_tsc)
> > +		kvm_x86_ops->set_hv_timer(vcpu, tscdeadline - guest_tsc);
> > +	else
> > +		kvm_x86_ops->set_hv_timer(vcpu, 0);
> 
> Calling kvm_lapic_expired_hv_timer will avoid the useless immediate
> vmexit after a vmentry.

Even if this is common enough to warrant optimizing for it (which I'm
not sure about, and surely depends on the workload), I think it should
be done later.  You'd get the same "useless vmentry" even with
hrtimer-based LAPIC timer.

Thanks,

Paolo