xen-hvm: ignore background I/O sections

Since Xen will correctly handle accesses to unimplemented I/O ports (by
returning all 1's for reads and ignoring writes) there is no need for
QEMU to register backgroud I/O sections.

This patch therefore adds checks to xen_io_add/del so that sections with
memory-region ops pointing at 'unassigned_io_ops' are ignored.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1462811480-16295-1-git-send-email-paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

xen-hvm.c

@@ -511,8 +511,13 @@ static void xen_io_add(MemoryListener *listener,
                        MemoryRegionSection *section)
 {
     XenIOState *state = container_of(listener, XenIOState, io_listener);
+    MemoryRegion *mr = section->mr;
 
-    memory_region_ref(section->mr);
+    if (mr->ops == &unassigned_io_ops) {
+        return;
+    }
+
+    memory_region_ref(mr);
 
     xen_map_io_section(xen_xc, xen_domid, state->ioservid, section);
 }
@@ -521,10 +526,15 @@ static void xen_io_del(MemoryListener *listener,
                        MemoryRegionSection *section)
 {
     XenIOState *state = container_of(listener, XenIOState, io_listener);
+    MemoryRegion *mr = section->mr;
+
+    if (mr->ops == &unassigned_io_ops) {
+        return;
+    }
 
     xen_unmap_io_section(xen_xc, xen_domid, state->ioservid, section);
 
-    memory_region_unref(section->mr);
+    memory_region_unref(mr);
 }
 
 static void xen_device_realize(DeviceListener *listener,