linux-kernel - [PATCH 5.4 06/50] KVM: remember position in kvm->vcpus array

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Date:   Fri, 24 Sep 2021 14:43:55 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCH 5.4 06/50] KVM: remember position in kvm->vcpus array

From: Radim Krčmář <rkrcmar@...hat.com>

commit 8750e72a79dda2f665ce17b62049f4d62130d991 upstream.

Fetching an index for any vcpu in kvm->vcpus array by traversing
the entire array everytime is costly.
This patch remembers the position of each vcpu in kvm->vcpus array
by storing it in vcpus_idx under kvm_vcpu structure.

Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
[borntraeger@...ibm.com]: backport to 4.19 (also fits for 5.4)
Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
Acked-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/kvm_host.h |   11 +++--------
 virt/kvm/kvm_main.c      |    5 +++--
 2 files changed, 6 insertions(+), 10 deletions(-)

--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -266,7 +266,8 @@ struct kvm_vcpu {
 	struct preempt_notifier preempt_notifier;
 #endif
 	int cpu;
-	int vcpu_id;
+	int vcpu_id; /* id given by userspace at creation */
+	int vcpu_idx; /* index in kvm->vcpus array */
 	int srcu_idx;
 	int mode;
 	u64 requests;
@@ -571,13 +572,7 @@ static inline struct kvm_vcpu *kvm_get_v
 
 static inline int kvm_vcpu_get_idx(struct kvm_vcpu *vcpu)
 {
-	struct kvm_vcpu *tmp;
-	int idx;
-
-	kvm_for_each_vcpu(idx, tmp, vcpu->kvm)
-		if (tmp == vcpu)
-			return idx;
-	BUG();
+	return vcpu->vcpu_idx;
 }
 
 #define kvm_for_each_memslot(memslot, slots)	\
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2864,7 +2864,8 @@ static int kvm_vm_ioctl_create_vcpu(stru
 		goto unlock_vcpu_destroy;
 	}
 
-	BUG_ON(kvm->vcpus[atomic_read(&kvm->online_vcpus)]);
+	vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
+	BUG_ON(kvm->vcpus[vcpu->vcpu_idx]);
 
 	/* Now it's all set up, let userspace reach it */
 	kvm_get_kvm(kvm);
@@ -2874,7 +2875,7 @@ static int kvm_vm_ioctl_create_vcpu(stru
 		goto unlock_vcpu_destroy;
 	}
 
-	kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
+	kvm->vcpus[vcpu->vcpu_idx] = vcpu;
 
 	/*
 	 * Pairs with smp_rmb() in kvm_get_vcpu.  Write kvm->vcpus