kern/include/ros/ring_buffer.h - upstream - Git at Google

 /******************************************************************************
  * ring.h
  *
  * Shared producer-consumer ring macros.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to
  * deal in the Software without restriction, including without limitation the
  * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
  * sell copies of the Software, and to permit persons to whom the Software is
  * furnished to do so, subject to the following conditions:
  *
  * The above copyright notice and this permission notice shall be included in
  * all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN THE SOFTWARE.
  *
  * Tim Deegan and Andrew Warfield November 2004.
  */

 #pragma once
 #include <string.h>
 #include <ros/arch/membar.h>

 #define xen_mb()  mb()
 #define xen_rmb() rmb()
 #define xen_wmb() wmb()

 /* Helpers for generic power-of-2 ring buffers */
 #define __ring_nr_empty(sz, prod, cons) ((sz) - ((prod) - (cons)))
 #define __ring_empty(prod, cons) ((prod) == (cons))
 #define __ring_nr_full(prod, cons) ((prod) - (cons))
 #define __ring_full(sz, prod, cons) (__ring_nr_empty((sz), (prod), (cons)) == 0)

 typedef unsigned int RING_IDX;

 /* Round a 32-bit unsigned constant down to the nearest power of two. */
 #define __RD2(_x)  (((_x) & 0x00000002UL) ? 0x2                  : ((_x) & 0x1))
 #define __RD4(_x)  (((_x) & 0x0000000cUL) ? __RD2((_x)>>2)<<2    : __RD2(_x))
 #define __RD8(_x)  (((_x) & 0x000000f0UL) ? __RD4((_x)>>4)<<4    : __RD4(_x))
 #define __RD16(_x) (((_x) & 0x0000ff00UL) ? __RD8((_x)>>8)<<8    : __RD8(_x))
 #define __RD32(_x) (((_x) & 0xffff0000UL) ? __RD16((_x)>>16)<<16 : __RD16(_x))

 /*
  * Calculate size of a shared ring, given the total available space for the
  * ring and indexes (_sz), and the name tag of the request/response structure.
  * A ring contains as many entries as will fit, rounded down to the nearest
  * power of two (so we can mask with (size-1) to loop around).
  */
 #define __CONST_RING_SIZE(_s, _sz) \
     (__RD32(((_sz) - offsetof(struct _s##_sring, ring)) / \
 	    sizeof(((struct _s##_sring *)0)->ring[0])))
 /*
  * The same for passing in an actual pointer instead of a name tag.
  */
 #define __RING_SIZE(_s, _sz) \
     (__RD32(((_sz) - (long)(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))

 /*
  * Macros to make the correct C datatypes for a new kind of ring.
  *
  * To make a new ring datatype, you need to have two message structures,
  * let's say request_t, and response_t already defined.
  *
  * In a header where you want the ring datatype declared, you then do:
  *
  *     DEFINE_RING_TYPES(mytag, request_t, response_t);
  *
  * These expand out to give you a set of types, as you can see below.
  * The most important of these are:
  *
  *     mytag_sring_t      - The shared ring.
  *     mytag_front_ring_t - The 'front' half of the ring.
  *     mytag_back_ring_t  - The 'back' half of the ring.
  *
  * To initialize a ring in your code you need to know the location and size
  * of the shared memory area (PAGE_SIZE, for instance). To initialise
  * the front half:
  *
  *     mytag_front_ring_t front_ring;
  *     SHARED_RING_INIT((mytag_sring_t *)shared_page);
  *     FRONT_RING_INIT(&front_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
  *
  * Initializing the back follows similarly (note that only the front
  * initializes the shared ring):
  *
  *     mytag_back_ring_t back_ring;
  *     BACK_RING_INIT(&back_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
  */

 #define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
                                                                         \
 /* Shared ring entry */                                                 \
 union __name##_sring_entry {                                            \
     __req_t req;                                                        \
     __rsp_t rsp;                                                        \
 };                                                                      \
                                                                         \
 /* Shared ring page */                                                  \
 struct __name##_sring {                                                 \
     RING_IDX req_prod, req_event;                                       \
     RING_IDX rsp_prod, rsp_event;                                       \
     union {                                                             \
         struct {                                                        \
             uint8_t smartpoll_active;                                   \
         } netif;                                                        \
         struct {                                                        \
             uint8_t msg;                                                \
         } tapif_user;                                                   \
         uint8_t pvt_pad[4];                                             \
     } priv;                                                             \
     uint8_t __pad[44];                                                  \
     union __name##_sring_entry ring[1]; /* variable-length */           \
 };                                                                      \
                                                                         \
 /* "Front" end's private variables */                                   \
 struct __name##_front_ring {                                            \
     RING_IDX req_prod_pvt;                                              \
     RING_IDX rsp_cons;                                                  \
     unsigned int nr_ents;                                               \
     struct __name##_sring *sring;                                       \
 };                                                                      \
                                                                         \
 /* "Back" end's private variables */                                    \
 struct __name##_back_ring {                                             \
     RING_IDX rsp_prod_pvt;                                              \
     RING_IDX req_cons;                                                  \
     unsigned int nr_ents;                                               \
     struct __name##_sring *sring;                                       \
 };                                                                      \
                                                                         \
 /* Syntactic sugar */                                                   \
 typedef struct __name##_sring __name##_sring_t;                         \
 typedef struct __name##_front_ring __name##_front_ring_t;               \
 typedef struct __name##_back_ring __name##_back_ring_t

 /*
  * Macros for manipulating rings.
  *
  * FRONT_RING_whatever works on the "front end" of a ring: here
  * requests are pushed on to the ring and responses taken off it.
  *
  * BACK_RING_whatever works on the "back end" of a ring: here
  * requests are taken off the ring and responses put on.
  *
  * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL.
  * This is OK in 1-for-1 request-response situations where the
  * requestor (front end) never has more than RING_SIZE()-1
  * outstanding requests.
  */

 /* Initialising empty rings */
 #define SHARED_RING_INIT(_s) do {                                       \
     (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
     (_s)->req_event = (_s)->rsp_event = 1;                              \
     (void)memset((_s)->priv.pvt_pad, 0, sizeof((_s)->priv.pvt_pad));    \
     (void)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;                                                 \
     (_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;                                                 \
     (_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)

 /* How big is this ring? */
 #define RING_SIZE(_r)                                                   \
     ((_r)->nr_ents)

 /* Number of free requests (for use on front side only). */
 #define RING_FREE_REQUESTS(_r)                                          \
     (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))

 /* Test if there is an empty slot available on the front ring.
  * (This is only meaningful from the front. )
  */
 #define RING_FULL(_r)                                                   \
     (RING_FREE_REQUESTS(_r) == 0)

 /* Test if there are outstanding messages to be processed on a ring. */
 #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
     ((_r)->sring->rsp_prod - (_r)->rsp_cons)

 #ifdef __GNUC__
 #define RING_HAS_UNCONSUMED_REQUESTS(_r) ({                             \
     unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;          \
     unsigned int rsp = RING_SIZE(_r) -                                  \
         ((_r)->req_cons - (_r)->rsp_prod_pvt);                          \
     req < rsp ? req : rsp;                                              \
 })
 #else
 /* Same as above, but without the nice GCC ({ ... }) syntax. */
 #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
     ((((_r)->sring->req_prod - (_r)->req_cons) <                        \
       (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt))) ?        \
      ((_r)->sring->req_prod - (_r)->req_cons) :                         \
      (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt)))
 #endif

 /* Direct access to individual ring elements, by index. */
 #define RING_GET_REQUEST(_r, _idx)                                      \
     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))

 #define RING_GET_RESPONSE(_r, _idx)                                     \
     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))

 /* Loop termination condition: Would the specified index overflow the ring? */
 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
     (((_cons) - (_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;                         \
 } while (0)

 #define RING_PUSH_RESPONSES(_r) do {                                    \
     xen_wmb(); /* front sees resps /before/ updated producer index */   \
     (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
 } while (0)

 /*
  * Notification hold-off (req_event and rsp_event):
  *
  * When queueing requests or responses on a shared ring, it may not always be
  * necessary to notify the remote end. For example, if requests are in flight
  * in a backend, the front may be able to queue further requests without
  * notifying the back (if the back checks for new requests when it queues
  * responses).
  *
  * When enqueuing requests or responses:
  *
  *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
  *  is a boolean return value. True indicates that the receiver requires an
  *  asynchronous notification.
  *
  * After dequeuing requests or responses (before sleeping the connection):
  *
  *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
  *  The second argument is a boolean return value. True indicates that there
  *  are pending messages on the ring (i.e., the connection should not be put
  *  to sleep).
  *
  *  These macros will set the req_event/rsp_event field to trigger a
  *  notification on the very next message that is enqueued. If you want to
  *  create batches of work (i.e., only receive a notification after several
  *  messages have been enqueued) then you will need to create a customised
  *  version of the FINAL_CHECK macro in your own code, which sets the event
  *  field appropriately.
  */

 #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
     RING_IDX __old = (_r)->sring->req_prod;                             \
     RING_IDX __new = (_r)->req_prod_pvt;                                \
     xen_wmb(); /* back sees requests /before/ updated producer index */ \
     (_r)->sring->req_prod = __new;                                      \
     xen_mb(); /* back sees new requests /before/ we check req_event */  \
     (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
                  (RING_IDX)(__new - __old));                            \
 } while (0)

 #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
     RING_IDX __old = (_r)->sring->rsp_prod;                             \
     RING_IDX __new = (_r)->rsp_prod_pvt;                                \
     xen_wmb(); /* front sees resps /before/ updated producer index */   \
     (_r)->sring->rsp_prod = __new;                                      \
     xen_mb(); /* front sees new resps /before/ we check rsp_event */    \
     (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
                  (RING_IDX)(__new - __old));                            \
 } while (0)

 #define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
     if (_work_to_do) break;                                             \
     (_r)->sring->req_event = (_r)->req_cons + 1;                        \
     xen_mb();                                                           \
     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 } while (0)

 #define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
     if (_work_to_do) break;                                             \
     (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
     xen_mb();                                                           \
     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 } while (0)

 /*
  * Local variables:
  * mode: C
  * c-set-style: "BSD"
  * c-basic-offset: 4
  * tab-width: 4
  * indent-tabs-mode: nil
  * End:
  */
	/******************************************************************************
	* ring.h
	*
	* Shared producer-consumer ring macros.
	*
	* Permission is hereby granted, free of charge, to any person obtaining a copy
	* of this software and associated documentation files (the "Software"), to
	* deal in the Software without restriction, including without limitation the
	* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
	* sell copies of the Software, and to permit persons to whom the Software is
	* furnished to do so, subject to the following conditions:
	*
	* The above copyright notice and this permission notice shall be included in
	* all copies or substantial portions of the Software.
	*
	* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
	* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
	* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
	* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
	* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
	* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
	* DEALINGS IN THE SOFTWARE.
	*
	* Tim Deegan and Andrew Warfield November 2004.
	*/

	#pragma once
	#include <string.h>
	#include <ros/arch/membar.h>

	#define xen_mb() mb()
	#define xen_rmb() rmb()
	#define xen_wmb() wmb()

	/* Helpers for generic power-of-2 ring buffers */
	#define __ring_nr_empty(sz, prod, cons) ((sz) - ((prod) - (cons)))
	#define __ring_empty(prod, cons) ((prod) == (cons))
	#define __ring_nr_full(prod, cons) ((prod) - (cons))
	#define __ring_full(sz, prod, cons) (__ring_nr_empty((sz), (prod), (cons)) == 0)

	typedef unsigned int RING_IDX;

	/* Round a 32-bit unsigned constant down to the nearest power of two. */
	#define __RD2(_x) (((_x) & 0x00000002UL) ? 0x2 : ((_x) & 0x1))
	#define __RD4(_x) (((_x) & 0x0000000cUL) ? __RD2((_x)>>2)<<2 : __RD2(_x))
	#define __RD8(_x) (((_x) & 0x000000f0UL) ? __RD4((_x)>>4)<<4 : __RD4(_x))
	#define __RD16(_x) (((_x) & 0x0000ff00UL) ? __RD8((_x)>>8)<<8 : __RD8(_x))
	#define __RD32(_x) (((_x) & 0xffff0000UL) ? __RD16((_x)>>16)<<16 : __RD16(_x))

	/*
	* Calculate size of a shared ring, given the total available space for the
	* ring and indexes (_sz), and the name tag of the request/response structure.
	* A ring contains as many entries as will fit, rounded down to the nearest
	* power of two (so we can mask with (size-1) to loop around).
	*/
	#define __CONST_RING_SIZE(_s, _sz) \
	(__RD32(((_sz) - offsetof(struct _s##_sring, ring)) / \
	sizeof(((struct _s##_sring *)0)->ring[0])))
	/*
	* The same for passing in an actual pointer instead of a name tag.
	*/
	#define __RING_SIZE(_s, _sz) \
	(__RD32(((_sz) - (long)(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))

	/*
	* Macros to make the correct C datatypes for a new kind of ring.
	*
	* To make a new ring datatype, you need to have two message structures,
	* let's say request_t, and response_t already defined.
	*
	* In a header where you want the ring datatype declared, you then do:
	*
	* DEFINE_RING_TYPES(mytag, request_t, response_t);
	*
	* These expand out to give you a set of types, as you can see below.
	* The most important of these are:
	*
	* mytag_sring_t - The shared ring.
	* mytag_front_ring_t - The 'front' half of the ring.
	* mytag_back_ring_t - The 'back' half of the ring.
	*
	* To initialize a ring in your code you need to know the location and size
	* of the shared memory area (PAGE_SIZE, for instance). To initialise
	* the front half:
	*
	* mytag_front_ring_t front_ring;
	* SHARED_RING_INIT((mytag_sring_t *)shared_page);
	* FRONT_RING_INIT(&front_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
	*
	* Initializing the back follows similarly (note that only the front
	* initializes the shared ring):
	*
	* mytag_back_ring_t back_ring;
	* BACK_RING_INIT(&back_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
	*/

	#define DEFINE_RING_TYPES(__name, __req_t, __rsp_t) \
	\
	/* Shared ring entry */ \
	union __name##_sring_entry { \
	__req_t req; \
	__rsp_t rsp; \
	}; \
	\
	/* Shared ring page */ \
	struct __name##_sring { \
	RING_IDX req_prod, req_event; \
	RING_IDX rsp_prod, rsp_event; \
	union { \
	struct { \
	uint8_t smartpoll_active; \
	} netif; \
	struct { \
	uint8_t msg; \
	} tapif_user; \
	uint8_t pvt_pad[4]; \
	} priv; \
	uint8_t __pad[44]; \
	union __name##_sring_entry ring[1]; /* variable-length */ \
	}; \
	\
	/* "Front" end's private variables */ \
	struct __name##_front_ring { \
	RING_IDX req_prod_pvt; \
	RING_IDX rsp_cons; \
	unsigned int nr_ents; \
	struct __name##_sring *sring; \
	}; \
	\
	/* "Back" end's private variables */ \
	struct __name##_back_ring { \
	RING_IDX rsp_prod_pvt; \
	RING_IDX req_cons; \
	unsigned int nr_ents; \
	struct __name##_sring *sring; \
	}; \
	\
	/* Syntactic sugar */ \
	typedef struct __name##_sring __name##_sring_t; \
	typedef struct __name##_front_ring __name##_front_ring_t; \
	typedef struct __name##_back_ring __name##_back_ring_t

	/*
	* Macros for manipulating rings.
	*
	* FRONT_RING_whatever works on the "front end" of a ring: here
	* requests are pushed on to the ring and responses taken off it.
	*
	* BACK_RING_whatever works on the "back end" of a ring: here
	* requests are taken off the ring and responses put on.
	*
	* N.B. these macros do NO INTERLOCKS OR FLOW CONTROL.
	* This is OK in 1-for-1 request-response situations where the
	* requestor (front end) never has more than RING_SIZE()-1
	* outstanding requests.
	*/

	/* Initialising empty rings */
	#define SHARED_RING_INIT(_s) do { \
	(_s)->req_prod = (_s)->rsp_prod = 0; \
	(_s)->req_event = (_s)->rsp_event = 1; \
	(void)memset((_s)->priv.pvt_pad, 0, sizeof((_s)->priv.pvt_pad)); \
	(void)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; \
	(_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; \
	(_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)

	/* How big is this ring? */
	#define RING_SIZE(_r) \
	((_r)->nr_ents)

	/* Number of free requests (for use on front side only). */
	#define RING_FREE_REQUESTS(_r) \
	(RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))

	/* Test if there is an empty slot available on the front ring.
	* (This is only meaningful from the front. )
	*/
	#define RING_FULL(_r) \
	(RING_FREE_REQUESTS(_r) == 0)

	/* Test if there are outstanding messages to be processed on a ring. */
	#define RING_HAS_UNCONSUMED_RESPONSES(_r) \
	((_r)->sring->rsp_prod - (_r)->rsp_cons)

	#ifdef __GNUC__
	#define RING_HAS_UNCONSUMED_REQUESTS(_r) ({ \
	unsigned int req = (_r)->sring->req_prod - (_r)->req_cons; \
	unsigned int rsp = RING_SIZE(_r) - \
	((_r)->req_cons - (_r)->rsp_prod_pvt); \
	req < rsp ? req : rsp; \
	})
	#else
	/* Same as above, but without the nice GCC ({ ... }) syntax. */
	#define RING_HAS_UNCONSUMED_REQUESTS(_r) \
	((((_r)->sring->req_prod - (_r)->req_cons) < \
	(RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt))) ? \
	((_r)->sring->req_prod - (_r)->req_cons) : \
	(RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt)))
	#endif

	/* Direct access to individual ring elements, by index. */
	#define RING_GET_REQUEST(_r, _idx) \
	(&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))

	#define RING_GET_RESPONSE(_r, _idx) \
	(&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))

	/* Loop termination condition: Would the specified index overflow the ring? */
	#define RING_REQUEST_CONS_OVERFLOW(_r, _cons) \
	(((_cons) - (_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; \
	} while (0)

	#define RING_PUSH_RESPONSES(_r) do { \
	xen_wmb(); /* front sees resps /before/ updated producer index */ \
	(_r)->sring->rsp_prod = (_r)->rsp_prod_pvt; \
	} while (0)

	/*
	* Notification hold-off (req_event and rsp_event):
	*
	* When queueing requests or responses on a shared ring, it may not always be
	* necessary to notify the remote end. For example, if requests are in flight
	* in a backend, the front may be able to queue further requests without
	* notifying the back (if the back checks for new requests when it queues
	* responses).
	*
	* When enqueuing requests or responses:
	*
	* Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
	* is a boolean return value. True indicates that the receiver requires an
	* asynchronous notification.
	*
	* After dequeuing requests or responses (before sleeping the connection):
	*
	* Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
	* The second argument is a boolean return value. True indicates that there
	* are pending messages on the ring (i.e., the connection should not be put
	* to sleep).
	*
	* These macros will set the req_event/rsp_event field to trigger a
	* notification on the very next message that is enqueued. If you want to
	* create batches of work (i.e., only receive a notification after several
	* messages have been enqueued) then you will need to create a customised
	* version of the FINAL_CHECK macro in your own code, which sets the event
	* field appropriately.
	*/

	#define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do { \
	RING_IDX __old = (_r)->sring->req_prod; \
	RING_IDX __new = (_r)->req_prod_pvt; \
	xen_wmb(); /* back sees requests /before/ updated producer index */ \
	(_r)->sring->req_prod = __new; \
	xen_mb(); /* back sees new requests /before/ we check req_event */ \
	(_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) < \
	(RING_IDX)(__new - __old)); \
	} while (0)

	#define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do { \
	RING_IDX __old = (_r)->sring->rsp_prod; \
	RING_IDX __new = (_r)->rsp_prod_pvt; \
	xen_wmb(); /* front sees resps /before/ updated producer index */ \
	(_r)->sring->rsp_prod = __new; \
	xen_mb(); /* front sees new resps /before/ we check rsp_event */ \
	(_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) < \
	(RING_IDX)(__new - __old)); \
	} while (0)

	#define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do { \
	(_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r); \
	if (_work_to_do) break; \
	(_r)->sring->req_event = (_r)->req_cons + 1; \
	xen_mb(); \
	(_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r); \
	} while (0)

	#define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do { \
	(_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r); \
	if (_work_to_do) break; \
	(_r)->sring->rsp_event = (_r)->rsp_cons + 1; \
	xen_mb(); \
	(_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r); \
	} while (0)

	/*
	* Local variables:
	* mode: C
	* c-set-style: "BSD"
	* c-basic-offset: 4
	* tab-width: 4
	* indent-tabs-mode: nil
	* End:
	*/