linux-kernel - [PATCH v3 24/26] KVM: x86: Skip APICv update if APICv is disable at the module level

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Message-Id: <20211208015236.1616697-25-seanjc@google.com>
Date:   Wed,  8 Dec 2021 01:52:34 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, Joerg Roedel <joro@...tes.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH v3 24/26] KVM: x86: Skip APICv update if APICv is disable at
 the module level

Bail from the APICv update paths _before_ taking apicv_update_lock if
APICv is disabled at the module level.  kvm_request_apicv_update() in
particular is invoked from multiple paths that can be reached without
APICv being enabled, e.g. svm_enable_irq_window(), and taking the
rw_sem for write when APICv is disabled may introduce unnecessary
contention and stalls.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/hyperv.c | 3 +++
 arch/x86/kvm/x86.c    | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 7179fa645eda..175c1bace091 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -112,6 +112,9 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 	if (!!auto_eoi_old == !!auto_eoi_new)
 		return;
 
+	if (!enable_apicv)
+		return;
+
 	down_write(&vcpu->kvm->arch.apicv_update_lock);
 
 	if (auto_eoi_new)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index abf99b77883e..c804cc39c90d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9602,6 +9602,9 @@ EXPORT_SYMBOL_GPL(__kvm_request_apicv_update);
 
 void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
+	if (!enable_apicv)
+		return;
+
 	down_write(&kvm->arch.apicv_update_lock);
 	__kvm_request_apicv_update(kvm, activate, bit);
 	up_write(&kvm->arch.apicv_update_lock);
-- 
2.34.1.400.ga245620fadb-goog