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

[Aili Yao]

On Thu, 9 Dec 2021 18:20:57 +0000
Sean Christopherson <seanjc@xxxxxxxxxx> wrote:

> On Wed, Dec 08, 2021, Aili Yao wrote:
> > On Tue, 7 Dec 2021 23:23:03 +0000
> > Sean Christopherson <seanjc@xxxxxxxxxx> 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