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
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