[v5 1/2] xen/ring: Add a new macro to detect whether there is an overflow in requests and response.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

We want to be able to exit if the difference between the request
produced (what the frontend tells us) and the requests consumed
(what we have so far processed) is greater than the ring size.

If so, we should terminate the loop as the request produced
is not trusted and it means it is bogus.

Cc: stable@vger.kernel.org
[v1: Make the macro do the ring thing, swap arguments around to
match the comment]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 include/xen/interface/io/ring.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
index 75271b9..94aa885 100644
--- a/include/xen/interface/io/ring.h
+++ b/include/xen/interface/io/ring.h
@@ -188,6 +188,12 @@ struct __name##_back_ring {						\
 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)				\
     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
 
+/* Loop termination condition: Is the difference between request produced
+ * and request consumed greater than the ring size. If so, terminate the
+ * loop. */
+#define RING_REQUEST_PROD_OVERFLOW(_r, _prod, _cons)                    \
+    (((_prod) - (_cons)) > RING_SIZE(_r))
+
 #define RING_PUSH_REQUESTS(_r) do {					\
     wmb(); /* back sees requests /before/ updated producer index */	\
     (_r)->sring->req_prod = (_r)->req_prod_pvt;				\
-- 
1.8.1.4