linux-kernel - [PATCH 3/3] KVM: LAPIC: Optimize PMI delivering overhead

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Date:   Fri,  8 Oct 2021 02:57:34 -0700
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH 3/3] KVM: LAPIC: Optimize PMI delivering overhead

From: Wanpeng Li <wanpengli@...cent.com>

The overhead of kvm_vcpu_kick() is huge since expensive rcu/memory
barrier etc operations in rcuwait_wake_up(). It is worse when local 
delivery since the vCPU is scheduled and we still suffer from this. 
We can observe 12us+ for kvm_vcpu_kick() in kvm_pmu_deliver_pmi() 
path by ftrace before the patch and 6us+ after the optimization. 

Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
 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 76fb00921203..ec6997187c6d 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1120,7 +1120,8 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
 	case APIC_DM_NMI:
 		result = 1;
 		kvm_inject_nmi(vcpu);
-		kvm_vcpu_kick(vcpu);
+		if (vcpu != kvm_get_running_vcpu())
+			kvm_vcpu_kick(vcpu);
 		break;
 
 	case APIC_DM_INIT:
-- 
2.25.1