Re: [PATCH v2] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt

Sean Christopherson
2021-12-09T18:21:14Z

On Wed, Dec 08, 2021, Aili Yao wrote:
> On Tue, 7 Dec 2021 23:23:03 +0000
> Sean Christopherson <seanjc@google.com> wrote:
>
> >
> > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> > {
> > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> > + (kvm_mwait_in_guest(vcpu) || kvm_hlt_in_guest(vcpu));
> > }
> >
> > bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> > {
> > - return kvm_x86_ops.set_hv_timer
> > - && !(kvm_mwait_in_guest(vcpu->kvm) ||
> > - kvm_can_post_timer_interrupt(vcpu));
> > + /*
> > + * Don't use the hypervisor timer, a.k.a. VMX Preemption Timer, if the
> > + * guest can execute MWAIT without exiting as the timer will stop
> > + * counting if the core enters C3 or lower. HLT in the guest is ok as
> > + * HLT is effectively C1 and the timer counts in C0, C1, and C2.
> > + *
> > + * Don't use the hypervisor timer if KVM can post a timer interrupt to
> > + * the guest since posted the timer avoids taking an extra a VM-Exit
> > + * when the timer expires.
> > + */
> > + return kvm_x86_ops.set_hv_timer &&
> > + !kvm_mwait_in_guest(vcpu->kvm) &&
> > + !kvm_can_post_timer_interrupt(vcpu));
> > }
> > EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);
> >
>
> Sorry, I am little confused here now:
> if kvm_can_post_timer_interrupt(vcpu) return true(cpu-pm enabled), then the kvm_can_use_hv_timer will always be false;
> if kvm_can_post_timer_interrupt(vcpu) return false(cpu-pm disable),then kvm_mwait_in_guest(vcpu->kvm) can't be true ether;
> It seems we don't need kvm_mwait_in_guest(vcpu->kvm) here?

We do, it's to prevent the guest from enter C3+ and stopping the VMX preemption
timer, e.g. if either kvm_vcpu_apicv_active() or pi_inject_timer evaluates false.