s390x/css: be more consistent if broken beyond repair

Calling do_subchannel_work with no function control flags set in SCSW is
a programming error. Currently we handle this differently in
do_subchannel_work_virtual and do_subchannel_work_passthrough. Let's be
consistent and guard with a common assert against this programming error.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Message-Id: <20171004154144.88995-2-pasic@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

hw/s390x/css.c

@@ -1245,44 +1245,36 @@ int do_subchannel_work_virtual(SubchDev *sch)
     } else if (s->ctrl & SCSW_FCTL_START_FUNC) {
         /* Triggered by both ssch and rsch. */
         sch_handle_start_func_virtual(sch);
-    } else {
-        /* Cannot happen. */
-        return 0;
     }
     css_inject_io_interrupt(sch);
     return 0;
 }
 
 int do_subchannel_work_passthrough(SubchDev *sch)
 {
-    int ret;
+    int ret = 0;
     SCSW *s = &sch->curr_status.scsw;
 
     if (s->ctrl & SCSW_FCTL_CLEAR_FUNC) {
         /* TODO: Clear handling */
         sch_handle_clear_func(sch);
-        ret = 0;
     } else if (s->ctrl & SCSW_FCTL_HALT_FUNC) {
         /* TODO: Halt handling */
         sch_handle_halt_func(sch);
-        ret = 0;
     } else if (s->ctrl & SCSW_FCTL_START_FUNC) {
         ret = sch_handle_start_func_passthrough(sch);
-    } else {
-        /* Cannot happen. */
-        return -ENODEV;
     }
 
     return ret;
 }
 
 static int do_subchannel_work(SubchDev *sch)
 {
-    if (sch->do_subchannel_work) {
-        return sch->do_subchannel_work(sch);
-    } else {
+    if (!sch->do_subchannel_work) {
         return -EINVAL;
     }
+    g_assert(sch->curr_status.scsw.ctrl & SCSW_CTRL_MASK_FCTL);
+    return sch->do_subchannel_work(sch);
 }
 
 static void copy_pmcw_to_guest(PMCW *dest, const PMCW *src)