KVM: PPC: Book3S HV: Don't do ultravisor calls on systems without ultravisor

Commit 22945688 ("KVM: PPC: Book3S HV: Support reset of secure
guest") added a call to uv_svm_terminate, which is an ultravisor
call, without any check that the guest is a secure guest or even that
the system has an ultravisor.  On a system without an ultravisor,
the ultracall will degenerate to a hypercall, but since we are not
in KVM guest context, the hypercall will get treated as a system
call, which could have random effects depending on what happens to
be in r0, and could also corrupt the current task's kernel stack.
Hence this adds a test for the guest being a secure guest before
doing uv_svm_terminate().

Fixes: 22945688 ("KVM: PPC: Book3S HV: Support reset of secure guest")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv.c

@@ -4983,7 +4983,8 @@ static void kvmppc_core_destroy_vm_hv(struct kvm *kvm)
 		if (nesting_enabled(kvm))
 			kvmhv_release_all_nested(kvm);
 		kvm->arch.process_table = 0;
-		uv_svm_terminate(kvm->arch.lpid);
+		if (kvm->arch.secure_guest)
+			uv_svm_terminate(kvm->arch.lpid);
 		kvmhv_set_ptbl_entry(kvm->arch.lpid, 0, 0);
 	}