Re: [PATCH] [backport for 4.19/5.4 stable] KVM: remember position in kvm->vcpus array

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On 21/09/21 15:48, Christian Borntraeger wrote:
From: Radim Krčmář <rkrcmar@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[borntraeger@xxxxxxxxxx]: backport to 4.19 (also fits for 5.4)
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
  include/linux/kvm_host.h | 11 +++--------
  virt/kvm/kvm_main.c      |  5 +++--
  2 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 8dd4ebb58e97..827f70ce0b49 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -248,7 +248,8 @@ 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;
@@ -551,13 +552,7 @@ static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
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) \
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a3d82113ae1c..86ef740763b5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2751,7 +2751,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
  		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);
@@ -2761,7 +2762,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
  		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


Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

The backport makes sense given the code in the stable branch now calls kvm_vcpu_get_idx more than it used to.

Paolo




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