[PATCH v3 06/10] KVM: Initialize a vCPU's index to '-1' while it's being created

[Sean Christopherson]

Invalidate a vCPU's index immediately after allocating storage for the vCPU
so that KVM doesn't incorrectly treat a vCPU that is the process of being
created as being vCPU0.  This will also allow detecting that a vCPU is in
the process of being created and thus otherwise unreachable, which is
useful for avoiding false positives in lockdep assertions on vcpu->mutex.

Unwind the index back to -1 if insert the vCPU into the array fails so that
kvm_arch_vcpu_destroy() sees the vCPU as unreachable, i.e. so that teardown
logic doesn't hit false positive lockdep assertions.  Opportunistically add
a comment to call out that the "real" index needs to be set before making
the vCPU visible to other tasks.

Note, kvm_wait_for_vcpu_online() naturally does the right thing thanks to
vcpu->vcpu_idx and kvm->online_vcpus being signed values.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e44c20c04961..98da4c889ffc 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4188,6 +4188,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 		goto vcpu_decrement;
 	}
 
+	vcpu->vcpu_idx = -1;
+
 	BUILD_BUG_ON(sizeof(struct kvm_run) > PAGE_SIZE);
 	page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
 	if (!page) {
@@ -4216,11 +4218,18 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 		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
+	 * to detect that the vCPU is unreachable, e.g. for lockdep asserts.
+	 */
 	vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
 	r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
 	WARN_ON_ONCE(r == -EBUSY);
-	if (r)
+	if (r) {
+		vcpu->vcpu_idx = -1;
 		goto unlock_vcpu_destroy;
+	}
 
 	/*
 	 * Now it's all set up, let userspace reach it.  Grab the vCPU's mutex
-- 
2.55.0.rc0.799.gd6f94ed593-goog