ring.h

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/******************************************************************************
 * ring.h
 *
 * Shared producer-consumer ring macros.
 *
 * Tim Deegan and Andrew Warfield November 2004.
 */

#ifndef __XEN_PUBLIC_IO_RING_H__
#define __XEN_PUBLIC_IO_RING_H__

typedef unsigned int RING_IDX;

/* Round a 32-bit unsigned constant down to the nearest power of two. */
#define __RD2(_x)  (((_x) & 0x00000002) ? 0x2              : ((_x) & 0x1))
#define __RD4(_x)  (((_x) & 0x0000000c) ? __RD2((_x)>>2)<<2    : __RD2(_x))
#define __RD8(_x)  (((_x) & 0x000000f0) ? __RD4((_x)>>4)<<4    : __RD4(_x))
#define __RD16(_x) (((_x) & 0x0000ff00) ? __RD8((_x)>>8)<<8    : __RD8(_x))
#define __RD32(_x) (((_x) & 0xffff0000) ? __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 struct request, and struct response already defined.
 *
 * In a header where you want the ring datatype declared, you then do:
 *
 *     DEFINE_RING_TYPES(mytag, struct request, struct response);
 *
 * These expand out to give you a set of types, as you can see below.
 * The most important of these are:
 *
 *     struct mytag_sring      - The shared ring.
 *     struct mytag_front_ring - The 'front' half of the ring.
 *     struct mytag_back_ring  - 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:
 *
 *     struct mytag_front_ring front_ring;
 *     SHARED_RING_INIT((struct mytag_sring *)shared_page);
 *     FRONT_RING_INIT(&front_ring, (struct mytag_sring *)shared_page,
 *             PAGE_SIZE);
 *
 * Initializing the back follows similarly (note that only the front
 * initializes the shared ring):
 *
 *     struct mytag_back_ring back_ring;
 *     BACK_RING_INIT(&back_ring, (struct mytag_sring *)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;                   \
    uint8_t  pad[48];                           \
    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;                   \
};

/*
 * 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;              \
    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)

#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;                      \
    })

/* 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 {                 \
    wmb(); /* back sees requests /before/ updated producer index */ \
    (_r)->sring->req_prod = (_r)->req_prod_pvt;             \
} while (0)

#define RING_PUSH_RESPONSES(_r) do {                    \
    wmb(); /* front sees responses /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;                \
    wmb(); /* back sees requests /before/ updated producer index */ \
    (_r)->sring->req_prod = __new;                  \
    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;                \
    wmb(); /* front sees responses /before/ updated producer index */   \
    (_r)->sring->rsp_prod = __new;                  \
    mb(); /* front sees new responses /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;            \
    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;            \
    mb();                               \
    (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);          \
} while (0)

#endif /* __XEN_PUBLIC_IO_RING_H__ */