[v2] ring/macro: 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.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[v1: Make sure we have both _prod and _cons in the macro, not just _prod)
---
 xen/include/public/io/ring.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/xen/include/public/io/ring.h b/xen/include/public/io/ring.h
index d9884f5..277d296 100644
--- a/xen/include/public/io/ring.h
+++ b/xen/include/public/io/ring.h
@@ -234,6 +234,12 @@ typedef struct __name##_back_ring __name##_back_ring_t
 #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, _cons, _prod)                     \
+    (((_prod) - (_cons)) > 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;                         \
-- 
1.8.1.4