[PATCH v2 5/7] KVM: Move check for existing vCPU ID to the top of vCPU creation

[Sean Christopherson <seanjc@google.com>]

Now that kvm->lock is held for the entirety of vCPU creation, check for a
conflicting vCPU ID at the begnning of vCPU creation, before the arch
precreate() hook is invoked.  This will allow reverting commit 97d65b544f48
("KVM: Check for duplicate vcpu_id as early as possible").  For now, keep
the redundant vcpu_ids tracking as a sanity check.

No functional change intended (absent KVM bugs, checking vcpu_ids and
walking kvm_get_vcpu_by_id() should yield the same result).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c17cc8dd371b..d5524ac8c5cf 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4182,7 +4182,10 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 	if (kvm->created_vcpus >= kvm->max_vcpus)
 		return -EINVAL;
 
-	if (test_bit(id, kvm->vcpu_ids))
+	if (kvm_get_vcpu_by_id(kvm, id))
+		return -EEXIST;
+
+	if (WARN_ON_ONCE(test_bit(id, kvm->vcpu_ids)))
 		return -EEXIST;
 
 	r = kvm_arch_vcpu_precreate(kvm, id);
@@ -4221,11 +4224,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 			goto arch_vcpu_destroy;
 	}
 
-	if (WARN_ON_ONCE(kvm_get_vcpu_by_id(kvm, id))) {
-		r = -EEXIST;
-		goto unlock_vcpu_destroy;
-	}
-
 	/*
 	 * Set the vCPU's index *before* the vCPU is reachable by other tasks.
 	 * Unwind the index back to -1 on failure so that KVM can use the index
-- 
2.55.0.1082.g2b9226bbc0-goog