diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 2b1f7c887a..eb868ab738 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -568,7 +568,7 @@ long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE(void) arg)
         else if ( IS_PRIV(current->domain) )
         {
             d = find_domain_by_id(a.domid);
-            if ( !d )
+            if ( d == NULL )
                 return -ESRCH;
         }
         else
@@ -578,22 +578,24 @@ long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE(void) arg)
 
         if ( op == HVMOP_set_param )
         {
-            rc = 0;
             d->arch.hvm_domain.params[a.index] = a.value;
+            rc = 0;
         }
         else
         {
-            rc = d->arch.hvm_domain.params[a.index];
+            a.value = d->arch.hvm_domain.params[a.index];
+            rc = copy_to_guest(arg, &a, 1) ? -EFAULT : 0;
         }
 
         put_domain(d);
-        return rc;
+        break;
     }
 
     default:
     {
         DPRINTK("Bad HVM op %ld.\n", op);
         rc = -ENOSYS;
+        break;
     }
     }