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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Wed, 19 Jan 2022 15:12:09 +0800
Message-ID: <CANRm+CxOvAo_bvMYOkdwRh8og4HxU3mRR94Jp0SO1+qEP2ak_g@mail.gmail.com>
Subject: Re: [PATCH v2] KVM: LAPIC: Enable timer posted-interrupt when
 mwait/hlt is advertised
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, Aili Yao <yaoaili@kingsoft.com>
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kindly ping, :)
On Thu, 6 Jan 2022 at 20:21, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> From: Wanpeng Li <wanpengli@tencent.com>
>
> As commit 0c5f81dad46 (KVM: LAPIC: Inject timer interrupt via posted interrupt)
> mentioned that the host admin should well tune the guest setup, so that vCPUs
> are placed on isolated pCPUs, and with several pCPUs surplus for *busy* housekeeping.
> It is better to disable mwait/hlt/pause vmexits to keep the vCPUs in non-root
> mode. However, we may isolate pCPUs for other purpose like DPDK or we can make
> some guests isolated and others not, we may lose vmx preemption timer/timer fastpath
> due to not well tuned setup, and the checking in kvm_can_post_timer_interrupt()
> is not enough. Let's guarantee mwait/hlt is advertised before enabling posted-interrupt
> interrupt. vmx preemption timer/timer fastpath can continue to work if both of them
> are not advertised.
>
> Reported-by: Aili Yao <yaoaili@kingsoft.com>
> Cc: Aili Yao <yaoaili@kingsoft.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v1 -> v2:
>  * also check kvm_hlt_in_guest since sometime mwait is disabled on host
>
>  arch/x86/kvm/lapic.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index f206fc3..fdb7c81 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -113,7 +113,8 @@ 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) || kvm_hlt_in_guest(vcpu->kvm));
>  }
>
>  bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> --
> 2.7.4
>