linux-kernel - [PATCH v2 4/4] KVM: x86: Use rw_semaphore for APICv lock to allow vCPU parallelism

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Date:   Thu, 21 Oct 2021 17:49:27 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH v2 4/4] KVM: x86: Use rw_semaphore for APICv lock to allow
 vCPU parallelism

Use a rw_semaphore instead of a mutex to coordinate APICv updates so that
vCPUs responding to requests can take the lock for read and run in
parallel.  Using a mutex forces serialization of vCPUs even though
kvm_vcpu_update_apicv() only touches data local to that vCPU or is
protected by a different lock, e.g. SVM's ir_list_lock.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/include/asm/kvm_host.h |  2 +-
 arch/x86/kvm/hyperv.c           |  4 ++--
 arch/x86/kvm/x86.c              | 12 +++++++-----
 3 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f64eef86391d..b61c03cda2b4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1070,7 +1070,7 @@ struct kvm_arch {
 	atomic_t apic_map_dirty;
 
 	/* Protects apic_access_memslot_enabled and apicv_inhibit_reasons */
-	struct mutex apicv_update_lock;
+	struct rw_semaphore apicv_update_lock;
 
 	bool apic_access_memslot_enabled;
 	unsigned long apicv_inhibit_reasons;
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 6f11cda2bfa4..4f15c0165c05 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -112,7 +112,7 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 	if (!!auto_eoi_old == !!auto_eoi_new)
 		return;
 
-	mutex_lock(&vcpu->kvm->arch.apicv_update_lock);
+	down_write(&vcpu->kvm->arch.apicv_update_lock);
 
 	if (auto_eoi_new)
 		hv->synic_auto_eoi_used++;
@@ -123,7 +123,7 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 				   !hv->synic_auto_eoi_used,
 				   APICV_INHIBIT_REASON_HYPERV);
 
-	mutex_unlock(&vcpu->kvm->arch.apicv_update_lock);
+	up_write(&vcpu->kvm->arch.apicv_update_lock);
 }
 
 static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 03103b69e8a6..77f6dc3aa4ee 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8742,7 +8742,7 @@ EXPORT_SYMBOL_GPL(kvm_apicv_activated);
 
 static void kvm_apicv_init(struct kvm *kvm)
 {
-	mutex_init(&kvm->arch.apicv_update_lock);
+	init_rwsem(&kvm->arch.apicv_update_lock);
 
 	if (enable_apicv)
 		clear_bit(APICV_INHIBIT_REASON_DISABLE,
@@ -9402,7 +9402,7 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu)
 	if (!lapic_in_kernel(vcpu))
 		return;
 
-	mutex_lock(&vcpu->kvm->arch.apicv_update_lock);
+	down_read(&vcpu->kvm->arch.apicv_update_lock);
 
 	activate = kvm_apicv_activated(vcpu->kvm);
 	if (apic->apicv_active == activate)
@@ -9422,7 +9422,7 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu)
 		kvm_make_request(KVM_REQ_EVENT, vcpu);
 
 out:
-	mutex_unlock(&vcpu->kvm->arch.apicv_update_lock);
+	up_read(&vcpu->kvm->arch.apicv_update_lock);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_update_apicv);
 
@@ -9430,6 +9430,8 @@ void __kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
 	unsigned long old, new;
 
+	lockdep_assert_held_write(&kvm->arch.apicv_update_lock);
+
 	if (!kvm_x86_ops.check_apicv_inhibit_reasons ||
 	    !static_call(kvm_x86_check_apicv_inhibit_reasons)(bit))
 		return;
@@ -9468,9 +9470,9 @@ EXPORT_SYMBOL_GPL(__kvm_request_apicv_update);
 
 void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
-	mutex_lock(&kvm->arch.apicv_update_lock);
+	down_write(&kvm->arch.apicv_update_lock);
 	__kvm_request_apicv_update(kvm, activate, bit);
-	mutex_unlock(&kvm->arch.apicv_update_lock);
+	up_write(&kvm->arch.apicv_update_lock);
 }
 EXPORT_SYMBOL_GPL(kvm_request_apicv_update);
 
-- 
2.33.0.1079.g6e70778dc9-goog