io/ring.h: new macro to detect whether there are too many requests on the ring

Backends may need to protect themselves against an insane number of
produced requests stored by a frontend, in case they iterate over
requests until reaching the req_prod value. There can't be more
requests on the ring than the difference between produced requests
and produced (but possibly not yet published) responses.

This is a more strict alternative to a patch previously posted by
Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/public/io/ring.h
+++ b/xen/include/public/io/ring.h
@@ -234,6 +234,10 @@ typedef struct __name##_back_ring __name
 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
 
+/* Ill-behaved frontend determination: Can there be this many requests? */
+#define RING_REQUEST_PROD_OVERFLOW(_r, _prod)                           \
+    (((_prod) - (_r)->rsp_prod_pvt) > RING_SIZE(_r))
+
 #define RING_PUSH_REQUESTS(_r) do {                                     \
     xen_wmb(); /* back sees requests /before/ updated producer index */ \
     (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \