virtio-ccw: diag 500 may return a negative cookie

cohuck · mstsirkin · commit 8457fdfeb16d · 2019-03-06T11:19:33.000-05:00
If something goes wrong in the kvm io bus handling, the virtio-ccw
diagnose may return a negative error value in the cookie gpr.

Document this.

Reviewed-by: Halil Pasic &lt;pasic@linux.ibm.com&gt;
Signed-off-by: Cornelia Huck &lt;cohuck@redhat.com&gt;
Signed-off-by: Michael S. Tsirkin &lt;mst@redhat.com&gt;
diff --git a/Documentation/virtual/kvm/s390-diag.txt b/Documentation/virtual/kvm/s390-diag.txt
@@ -68,7 +68,8 @@ Subcode 3 - virtio-ccw notification
     identifier, it is ignored.
 
     After completion of the DIAGNOSE call, general register 2 may contain
-    a 64bit identifier (in the kvm_io_bus cookie case).
+    a 64bit identifier (in the kvm_io_bus cookie case), or a negative
+    error value, if an internal error occurred.
 
     See also the virtio standard for a discussion of this hypercall.
 
