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   1 /******************************************************************************
   2  * ring.h
   3  *
   4  * Shared producer-consumer ring macros.
   5  *
   6  * Tim Deegan and Andrew Warfield November 2004.
   7  */
   8
   9 #ifndef __XEN_PUBLIC_IO_RING_H__
  10 #define __XEN_PUBLIC_IO_RING_H__
  11
  12 typedef unsigned int RING_IDX;
  13
  14 /* Round a 32-bit unsigned constant down to the nearest power of two. */
  15 #define __RD2(_x)  (((_x) & 0x00000002) ? 0x2                  : ((_x) & 0x1))
  16 #define __RD4(_x)  (((_x) & 0x0000000c) ? __RD2((_x)>>2)<<2    : __RD2(_x))
  17 #define __RD8(_x)  (((_x) & 0x000000f0) ? __RD4((_x)>>4)<<4    : __RD4(_x))
  18 #define __RD16(_x) (((_x) & 0x0000ff00) ? __RD8((_x)>>8)<<8    : __RD8(_x))
  19 #define __RD32(_x) (((_x) & 0xffff0000) ? __RD16((_x)>>16)<<16 : __RD16(_x))
  20
  21 /*
  22  * Calculate size of a shared ring, given the total available space for the
  23  * ring and indexes (_sz), and the name tag of the request/response structure.
  24  * A ring contains as many entries as will fit, rounded down to the nearest
  25  * power of two (so we can mask with (size-1) to loop around).
  26  */
  27 #define __RING_SIZE(_s, _sz) \
  28     (__RD32(((_sz) - (long)&(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))
  29
  30 /*
  31  * Macros to make the correct C datatypes for a new kind of ring.
  32  *
  33  * To make a new ring datatype, you need to have two message structures,
  34  * let's say struct request, and struct response already defined.
  35  *
  36  * In a header where you want the ring datatype declared, you then do:
  37  *
  38  *     DEFINE_RING_TYPES(mytag, struct request, struct response);
  39  *
  40  * These expand out to give you a set of types, as you can see below.
  41  * The most important of these are:
  42  *
  43  *     struct mytag_sring      - The shared ring.
  44  *     struct mytag_front_ring - The 'front' half of the ring.
  45  *     struct mytag_back_ring  - The 'back' half of the ring.
  46  *
  47  * To initialize a ring in your code you need to know the location and size
  48  * of the shared memory area (PAGE_SIZE, for instance). To initialise
  49  * the front half:
  50  *
  51  *     struct mytag_front_ring front_ring;
  52  *     SHARED_RING_INIT((struct mytag_sring *)shared_page);
  53  *     FRONT_RING_INIT(&front_ring, (struct mytag_sring *)shared_page,
  54  *                     PAGE_SIZE);
  55  *
  56  * Initializing the back follows similarly (note that only the front
  57  * initializes the shared ring):
  58  *
  59  *     struct mytag_back_ring back_ring;
  60  *     BACK_RING_INIT(&back_ring, (struct mytag_sring *)shared_page,
  61  *                    PAGE_SIZE);
  62  */
  63
  64 #define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
  65                                                                         \
  66 /* Shared ring entry */                                                 \
  67 union __name##_sring_entry {                                            \
  68     __req_t req;                                                        \
  69     __rsp_t rsp;                                                        \
  70 };                                                                      \
  71                                                                         \
  72 /* Shared ring page */                                                  \
  73 struct __name##_sring {                                                 \
  74     RING_IDX req_prod, req_event;                                       \
  75     RING_IDX rsp_prod, rsp_event;                                       \
  76     uint8_t  pad[48];                                                   \
  77     union __name##_sring_entry ring[1]; /* variable-length */           \
  78 };                                                                      \
  79                                                                         \
  80 /* "Front" end's private variables */                                   \
  81 struct __name##_front_ring {                                            \
  82     RING_IDX req_prod_pvt;                                              \
  83     RING_IDX rsp_cons;                                                  \
  84     unsigned int nr_ents;                                               \
  85     struct __name##_sring *sring;                                       \
  86 };                                                                      \
  87                                                                         \
  88 /* "Back" end's private variables */                                    \
  89 struct __name##_back_ring {                                             \
  90     RING_IDX rsp_prod_pvt;                                              \
  91     RING_IDX req_cons;                                                  \
  92     unsigned int nr_ents;                                               \
  93     struct __name##_sring *sring;                                       \
  94 };
  95
  96 /*
  97  * Macros for manipulating rings.
  98  *
  99  * FRONT_RING_whatever works on the "front end" of a ring: here
 100  * requests are pushed on to the ring and responses taken off it.
 101  *
 102  * BACK_RING_whatever works on the "back end" of a ring: here
 103  * requests are taken off the ring and responses put on.
 104  *
 105  * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL.
 106  * This is OK in 1-for-1 request-response situations where the
 107  * requestor (front end) never has more than RING_SIZE()-1
 108  * outstanding requests.
 109  */
 110
 111 /* Initialising empty rings */
 112 #define SHARED_RING_INIT(_s) do {                                       \
 113     (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
 114     (_s)->req_event = (_s)->rsp_event = 1;                              \
 115     memset((_s)->pad, 0, sizeof((_s)->pad));                            \
 116 } while(0)
 117
 118 #define FRONT_RING_INIT(_r, _s, __size) do {                            \
 119     (_r)->req_prod_pvt = 0;                                             \
 120     (_r)->rsp_cons = 0;                                                 \
 121     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 122     (_r)->sring = (_s);                                                 \
 123 } while (0)
 124
 125 #define BACK_RING_INIT(_r, _s, __size) do {                             \
 126     (_r)->rsp_prod_pvt = 0;                                             \
 127     (_r)->req_cons = 0;                                                 \
 128     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 129     (_r)->sring = (_s);                                                 \
 130 } while (0)
 131
 132 /* Initialize to existing shared indexes -- for recovery */
 133 #define FRONT_RING_ATTACH(_r, _s, __size) do {                          \
 134     (_r)->sring = (_s);                                                 \
 135     (_r)->req_prod_pvt = (_s)->req_prod;                                \
 136     (_r)->rsp_cons = (_s)->rsp_prod;                                    \
 137     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 138 } while (0)
 139
 140 #define BACK_RING_ATTACH(_r, _s, __size) do {                           \
 141     (_r)->sring = (_s);                                                 \
 142     (_r)->rsp_prod_pvt = (_s)->rsp_prod;                                \
 143     (_r)->req_cons = (_s)->req_prod;                                    \
 144     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 145 } while (0)
 146
 147 /* How big is this ring? */
 148 #define RING_SIZE(_r)                                                   \
 149     ((_r)->nr_ents)
 150
 151 /* Number of free requests (for use on front side only). */
 152 #define RING_FREE_REQUESTS(_r)                                          \
 153     (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))
 154
 155 /* Test if there is an empty slot available on the front ring.
 156  * (This is only meaningful from the front. )
 157  */
 158 #define RING_FULL(_r)                                                   \
 159     (RING_FREE_REQUESTS(_r) == 0)
 160
 161 /* Test if there are outstanding messages to be processed on a ring. */
 162 #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
 163     ((_r)->sring->rsp_prod - (_r)->rsp_cons)
 164
 165 #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
 166     ({                                                                  \
 167         unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;      \
 168         unsigned int rsp = RING_SIZE(_r) -                              \
 169                            ((_r)->req_cons - (_r)->rsp_prod_pvt);       \
 170         req < rsp ? req : rsp;                                          \
 171     })
 172
 173 /* Direct access to individual ring elements, by index. */
 174 #define RING_GET_REQUEST(_r, _idx)                                      \
 175     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
 176
 177 #define RING_GET_RESPONSE(_r, _idx)                                     \
 178     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
 179
 180 /* Loop termination condition: Would the specified index overflow the ring? */
 181 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
 182     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
 183
 184 #define RING_PUSH_REQUESTS(_r) do {                                     \
 185     wmb(); /* back sees requests /before/ updated producer index */     \
 186     (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \
 187 } while (0)
 188
 189 #define RING_PUSH_RESPONSES(_r) do {                                    \
 190     wmb(); /* front sees responses /before/ updated producer index */   \
 191     (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
 192 } while (0)
 193
 194 /*
 195  * Notification hold-off (req_event and rsp_event):
 196  *
 197  * When queueing requests or responses on a shared ring, it may not always be
 198  * necessary to notify the remote end. For example, if requests are in flight
 199  * in a backend, the front may be able to queue further requests without
 200  * notifying the back (if the back checks for new requests when it queues
 201  * responses).
 202  *
 203  * When enqueuing requests or responses:
 204  *
 205  *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
 206  *  is a boolean return value. True indicates that the receiver requires an
 207  *  asynchronous notification.
 208  *
 209  * After dequeuing requests or responses (before sleeping the connection):
 210  *
 211  *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
 212  *  The second argument is a boolean return value. True indicates that there
 213  *  are pending messages on the ring (i.e., the connection should not be put
 214  *  to sleep).
 215  *
 216  *  These macros will set the req_event/rsp_event field to trigger a
 217  *  notification on the very next message that is enqueued. If you want to
 218  *  create batches of work (i.e., only receive a notification after several
 219  *  messages have been enqueued) then you will need to create a customised
 220  *  version of the FINAL_CHECK macro in your own code, which sets the event
 221  *  field appropriately.
 222  */
 223
 224 #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
 225     RING_IDX __old = (_r)->sring->req_prod;                             \
 226     RING_IDX __new = (_r)->req_prod_pvt;                                \
 227     wmb(); /* back sees requests /before/ updated producer index */     \
 228     (_r)->sring->req_prod = __new;                                      \
 229     mb(); /* back sees new requests /before/ we check req_event */      \
 230     (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
 231                  (RING_IDX)(__new - __old));                            \
 232 } while (0)
 233
 234 #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
 235     RING_IDX __old = (_r)->sring->rsp_prod;                             \
 236     RING_IDX __new = (_r)->rsp_prod_pvt;                                \
 237     wmb(); /* front sees responses /before/ updated producer index */   \
 238     (_r)->sring->rsp_prod = __new;                                      \
 239     mb(); /* front sees new responses /before/ we check rsp_event */    \
 240     (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
 241                  (RING_IDX)(__new - __old));                            \
 242 } while (0)
 243
 244 #define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
 245     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 246     if (_work_to_do) break;                                             \
 247     (_r)->sring->req_event = (_r)->req_cons + 1;                        \
 248     mb();                                                               \
 249     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 250 } while (0)
 251
 252 #define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
 253     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 254     if (_work_to_do) break;                                             \
 255     (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
 256     mb();                                                               \
 257     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 258 } while (0)
 259
 260 #endif /* __XEN_PUBLIC_IO_RING_H__ */