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SubjectRe: [PATCH v2] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt
On Thu, 9 Dec 2021 18:20:57 +0000
Sean Christopherson <seanjc@google.com> wrote:

> 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.

Great thanks for your explanation!
Now i am clear!

--Aili Yao

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