KVM: SVM: Hide SEV migration lockdep goo behind CONFIG_PROVE_LOCKING

Wrap the manipulation of @role and the manual mutex_{release,acquire}()
invocations in CONFIG_PROVE_LOCKING=y to squash a clang-15 warning.  When
building with -Wunused-but-set-parameter and CONFIG_DEBUG_LOCK_ALLOC=n,
clang-15 seees there's no usage of @role in mutex_lock_killable_nested()
and yells.  PROVE_LOCKING selects DEBUG_LOCK_ALLOC, and the only reason
KVM manipulates @role is to make PROVE_LOCKING happy.

To avoid true ugliness, use "i" and "j" to detect the first pass in the
loops; the "idx" field that's used by kvm_for_each_vcpu() is guaranteed
to be '0' on the first pass as it's simply the first entry in the vCPUs
XArray, which is fully KVM controlled.  kvm_for_each_vcpu() passes '0'
for xa_for_each_range()'s "start", and xa_for_each_range() will not enter
the loop if there's no entry at '0'.

Fixes: 0c2c7c06 ("KVM: SEV: Mark nested locking of vcpu->lock")
Reported-by: kernel test robot <lkp@intel.com>
Cc: Peter Gonda <pgonda@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220613214237.2538266-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -1606,38 +1606,35 @@ static int sev_lock_vcpus_for_migration(struct kvm *kvm,
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i, j;
-	bool first = true;
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (mutex_lock_killable_nested(&vcpu->mutex, role))
 			goto out_unlock;
 
-		if (first) {
+#ifdef CONFIG_PROVE_LOCKING
+		if (!i)
 			/*
 			 * Reset the role to one that avoids colliding with
 			 * the role used for the first vcpu mutex.
 			 */
 			role = SEV_NR_MIGRATION_ROLES;
-			first = false;
-		} else {
+		else
 			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
-		}
+#endif
 	}
 
 	return 0;
 
 out_unlock:
 
-	first = true;
 	kvm_for_each_vcpu(j, vcpu, kvm) {
 		if (i == j)
 			break;
 
-		if (first)
-			first = false;
-		else
+#ifdef CONFIG_PROVE_LOCKING
+		if (j)
 			mutex_acquire(&vcpu->mutex.dep_map, role, 0, _THIS_IP_);
-
+#endif
 
 		mutex_unlock(&vcpu->mutex);
 	}