Date:   Thu, 9 Dec 2021 18:20:57 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Aili Yao <yaoaili126@gmail.com>
Cc:     pbonzini@redhat.com, vkuznets@redhat.com, wanpengli@tencent.com,
        jmattson@google.com, joro@8bytes.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, hpa@zytor.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, yaoaili@kingsoft.com
Subject: Re: [PATCH v2] KVM: LAPIC: Per vCPU control over
 kvm_can_post_timer_interrupt
Message-ID: <YbJJCf20VdHNnpzY@google.com>
References: <20211124125409.6eec3938@gmail.com>
 <Ya/s17QDlGZi9COR@google.com>
 <20211208182158.571fcdee@gmail.com>
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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.