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

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Date:   Thu, 16 Dec 2021 16:23:03 +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:
> On Tue, Nov 23, 2021 at 10:00 PM Wanpeng Li <kernellwp@...il.com> wrote:
> > ---
> >  arch/x86/kvm/lapic.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 759952dd1222..8257566d44c7 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -113,14 +113,13 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> >
> >  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_mwait_in_guest(vcpu->kvm) && kvm_vcpu_apicv_active(vcpu);  
> 
> As Aili's changelog pointed out, MWAIT may not be advertised to the guest. 
> 
> So I think we want this?  With a non-functional, opinionated refactoring of
> kvm_can_use_hv_timer() because I'm terrible at reading !(a || b).
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 40270d7bc597..c77cb386d03d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -113,14 +113,25 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> 
>  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);
> 

It seems Sean and Wanpeng are busy with some other more important issues;
So Please let me try to merge Sean, Wanpeng's ideas and suggestions together,also including my opinions
into one possible approach and get it reviewed, Only if others are OK with this;

I will post a new patch for this later today or tomorrow.

Thanks!

--Aili Yao