s390x/css: disabled subchannels cannot be status pending

The 3270 code will try to post an attention interrupt when the
3270 emulator (e.g. x3270) attaches. If the guest has not yet
enabled the subchannel for the 3270 device, we will present a spurious
cc 1 (status pending) when it uses msch on it later on, e.g. when
trying to enable the subchannel.

To fix this, just don't do anything in css_conditional_io_interrupt()
if the subchannel is not enabled. The 3270 code will work fine with
that, and the other user of this function (virtio-ccw) never
attempts to post an interrupt for a disabled device to begin with.

CC: qemu-stable@nongnu.org
Reported-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

hw/s390x/css.c

@@ -616,6 +616,14 @@ void css_inject_io_interrupt(SubchDev *sch)
 
 void css_conditional_io_interrupt(SubchDev *sch)
 {
+    /*
+     * If the subchannel is not enabled, it is not made status pending
+     * (see PoP p. 16-17, "Status Control").
+     */
+    if (!(sch->curr_status.pmcw.flags & PMCW_FLAGS_MASK_ENA)) {
+        return;
+    }
+
     /*
      * If the subchannel is not currently status pending, make it pending
      * with alert status.