KVM: x86: Check for existing Hyper-V vCPU in kvm_hv_vcpu_init()

When potentially allocating/initializing the Hyper-V vCPU struct, check
for an existing instance in kvm_hv_vcpu_init() instead of requiring
callers to perform the check.  Relying on callers to do the check is
risky as it's all too easy for KVM to overwrite vcpu->arch.hyperv and
leak memory, and it adds additional burden on callers without much
benefit.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20220830133737.1539624-5-vkuznets@redhat.comSigned-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -936,9 +936,12 @@ static void stimer_init(struct kvm_vcpu_hv_stimer *stimer, int timer_index)
 
 static int kvm_hv_vcpu_init(struct kvm_vcpu *vcpu)
 {
-	struct kvm_vcpu_hv *hv_vcpu;
+	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
 	int i;
 
+	if (hv_vcpu)
+		return 0;
+
 	hv_vcpu = kzalloc(sizeof(struct kvm_vcpu_hv), GFP_KERNEL_ACCOUNT);
 	if (!hv_vcpu)
 		return -ENOMEM;
@@ -962,11 +965,9 @@ int kvm_hv_activate_synic(struct kvm_vcpu *vcpu, bool dont_zero_synic_pages)
 	struct kvm_vcpu_hv_synic *synic;
 	int r;
 
-	if (!to_hv_vcpu(vcpu)) {
 	r = kvm_hv_vcpu_init(vcpu);
 	if (r)
 		return r;
-	}
 
 	synic = to_hv_synic(vcpu);
 
@@ -1660,10 +1661,8 @@ int kvm_hv_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data, bool host)
 	if (!host && !vcpu->arch.hyperv_enabled)
 		return 1;
 
-	if (!to_hv_vcpu(vcpu)) {
 	if (kvm_hv_vcpu_init(vcpu))
 		return 1;
-	}
 
 	if (kvm_hv_msr_partition_wide(msr)) {
 		int r;
@@ -1683,10 +1682,8 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
 	if (!host && !vcpu->arch.hyperv_enabled)
 		return 1;
 
-	if (!to_hv_vcpu(vcpu)) {
 	if (kvm_hv_vcpu_init(vcpu))
 		return 1;
-	}
 
 	if (kvm_hv_msr_partition_wide(msr)) {
 		int r;
@@ -2000,7 +1997,7 @@ void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu)
 		return;
 	}
 
-	if (!to_hv_vcpu(vcpu) && kvm_hv_vcpu_init(vcpu))
+	if (kvm_hv_vcpu_init(vcpu))
 		return;
 
 	hv_vcpu = to_hv_vcpu(vcpu);