linux-kernel - Re: [PATCH v2] KVM: LAPIC: Per vCPU control over kvm_can_post_timer

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Date:   Wed, 8 Dec 2021 18:21:58 +0800
From:   Aili Yao <yaoaili126@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, yaoaili@...gsoft.com
Subject: Re: [PATCH v2] KVM: LAPIC: Per vCPU control over
 kvm_can_post_timer_interrupt

On Tue, 7 Dec 2021 23:23:03 +0000
Sean Christopherson <seanjc@...gle.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?

Sorry, I am just a little confused and not too sure about this, if anything wrong, just ignore it.

Thanks!