xen/interface: re-define FRONT/BACK_RING_ATTACH()

Currently these macros are defined to re-initialize a front/back ring
(respectively) to values read from the shared ring in such a way that any
requests/responses that are added to the shared ring whilst the front/back
is detached will be skipped over. This, in general, is not a desirable
semantic since most frontend implementations will eventually block waiting
for a response which would either never appear or never be processed.

Since the macros are currently unused, take this opportunity to re-define
them to re-initialize a front/back ring using specified values. This also
allows FRONT/BACK_RING_INIT() to be re-defined in terms of
FRONT/BACK_RING_ATTACH() using a specified value of 0.

NOTE: BACK_RING_ATTACH() will be used directly in a subsequent patch.

Signed-off-by: Paul Durrant <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

include/xen/interface/io/ring.h

···
     memset((_s)->pad, 0, sizeof((_s)->pad));				\
 } while(0)
 
-#define FRONT_RING_INIT(_r, _s, __size) do {				\
-    (_r)->req_prod_pvt = 0;						\
-    (_r)->rsp_cons = 0;							\
+#define FRONT_RING_ATTACH(_r, _s, _i, __size) do {			\
+    (_r)->req_prod_pvt = (_i);						\
+    (_r)->rsp_cons = (_i);						\
     (_r)->nr_ents = __RING_SIZE(_s, __size);				\
     (_r)->sring = (_s);							\
 } while (0)
 
-#define BACK_RING_INIT(_r, _s, __size) do {				\
-    (_r)->rsp_prod_pvt = 0;						\
-    (_r)->req_cons = 0;							\
+#define FRONT_RING_INIT(_r, _s, __size) FRONT_RING_ATTACH(_r, _s, 0, __size)
+
+#define BACK_RING_ATTACH(_r, _s, _i, __size) do {			\
+    (_r)->rsp_prod_pvt = (_i);						\
+    (_r)->req_cons = (_i);						\
     (_r)->nr_ents = __RING_SIZE(_s, __size);				\
     (_r)->sring = (_s);							\
 } while (0)
 
-/* Initialize to existing shared indexes -- for recovery */
-#define FRONT_RING_ATTACH(_r, _s, __size) do {				\
-    (_r)->sring = (_s);							\
-    (_r)->req_prod_pvt = (_s)->req_prod;				\
-    (_r)->rsp_cons = (_s)->rsp_prod;					\
-    (_r)->nr_ents = __RING_SIZE(_s, __size);				\
-} while (0)
-
-#define BACK_RING_ATTACH(_r, _s, __size) do {				\
-    (_r)->sring = (_s);							\
-    (_r)->rsp_prod_pvt = (_s)->rsp_prod;				\
-    (_r)->req_cons = (_s)->req_prod;					\
-    (_r)->nr_ents = __RING_SIZE(_s, __size);				\
-} while (0)
+#define BACK_RING_INIT(_r, _s, __size) BACK_RING_ATTACH(_r, _s, 0, __size)
 
 /* How big is this ring? */
 #define RING_SIZE(_r)							\