include/xen/interface/io/ring.h

   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 request_t, and response_t already defined.
  35  *
  36  * In a header where you want the ring datatype declared, you then do:
  37  *
  38  *     DEFINE_RING_TYPES(mytag, request_t, response_t);
  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  *     mytag_sring_t      - The shared ring.
  44  *     mytag_front_ring_t - The 'front' half of the ring.
  45  *     mytag_back_ring_t  - 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  *     mytag_front_ring_t front_ring;
  52  *     SHARED_RING_INIT((mytag_sring_t *)shared_page);
  53  *     FRONT_RING_INIT(&front_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
  54  *
  55  * Initializing the back follows similarly (note that only the front
  56  * initializes the shared ring):
  57  *
  58  *     mytag_back_ring_t back_ring;
  59  *     BACK_RING_INIT(&back_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
  60  */
  61
  62 #define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
  63                                                                         \
  64 /* Shared ring entry */                                                 \
  65 union __name##_sring_entry {                                            \
  66     __req_t req;                                                        \
  67     __rsp_t rsp;                                                        \
  68 };                                                                      \
  69                                                                         \
  70 /* Shared ring page */                                                  \
  71 struct __name##_sring {                                                 \
  72     RING_IDX req_prod, req_event;                                       \
  73     RING_IDX rsp_prod, rsp_event;                                       \
  74     uint8_t  pad[48];                                                   \
  75     union __name##_sring_entry ring[1]; /* variable-length */           \
  76 };                                                                      \
  77                                                                         \
  78 /* "Front" end's private variables */                                   \
  79 struct __name##_front_ring {                                            \
  80     RING_IDX req_prod_pvt;                                              \
  81     RING_IDX rsp_cons;                                                  \
  82     unsigned int nr_ents;                                               \
  83     struct __name##_sring *sring;                                       \
  84 };                                                                      \
  85                                                                         \
  86 /* "Back" end's private variables */                                    \
  87 struct __name##_back_ring {                                             \
  88     RING_IDX rsp_prod_pvt;                                              \
  89     RING_IDX req_cons;                                                  \
  90     unsigned int nr_ents;                                               \
  91     struct __name##_sring *sring;                                       \
  92 };                                                                      \
  93                                                                         \
  94 /* Syntactic sugar */                                                   \
  95 typedef struct __name##_sring __name##_sring_t;                         \
  96 typedef struct __name##_front_ring __name##_front_ring_t;               \
  97 typedef struct __name##_back_ring __name##_back_ring_t
  98
  99 /*
 100  * Macros for manipulating rings.
 101  *
 102  * FRONT_RING_whatever works on the "front end" of a ring: here
 103  * requests are pushed on to the ring and responses taken off it.
 104  *
 105  * BACK_RING_whatever works on the "back end" of a ring: here
 106  * requests are taken off the ring and responses put on.
 107  *
 108  * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL.
 109  * This is OK in 1-for-1 request-response situations where the
 110  * requestor (front end) never has more than RING_SIZE()-1
 111  * outstanding requests.
 112  */
 113
 114 /* Initialising empty rings */
 115 #define SHARED_RING_INIT(_s) do {                                       \
 116     (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
 117     (_s)->req_event = (_s)->rsp_event = 1;                              \
 118     memset((_s)->pad, 0, sizeof((_s)->pad));                            \
 119 } while(0)
 120
 121 #define FRONT_RING_INIT(_r, _s, __size) do {                            \
 122     (_r)->req_prod_pvt = 0;                                             \
 123     (_r)->rsp_cons = 0;                                                 \
 124     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 125     (_r)->sring = (_s);                                                 \
 126 } while (0)
 127
 128 #define BACK_RING_INIT(_r, _s, __size) do {                             \
 129     (_r)->rsp_prod_pvt = 0;                                             \
 130     (_r)->req_cons = 0;                                                 \
 131     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 132     (_r)->sring = (_s);                                                 \
 133 } while (0)
 134
 135 /* Initialize to existing shared indexes -- for recovery */
 136 #define FRONT_RING_ATTACH(_r, _s, __size) do {                          \
 137     (_r)->sring = (_s);                                                 \
 138     (_r)->req_prod_pvt = (_s)->req_prod;                                \
 139     (_r)->rsp_cons = (_s)->rsp_prod;                                    \
 140     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 141 } while (0)
 142
 143 #define BACK_RING_ATTACH(_r, _s, __size) do {                           \
 144     (_r)->sring = (_s);                                                 \
 145     (_r)->rsp_prod_pvt = (_s)->rsp_prod;                                \
 146     (_r)->req_cons = (_s)->req_prod;                                    \
 147     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 148 } while (0)
 149
 150 /* How big is this ring? */
 151 #define RING_SIZE(_r)                                                   \
 152     ((_r)->nr_ents)
 153
 154 /* Number of free requests (for use on front side only). */
 155 #define RING_FREE_REQUESTS(_r)                                          \
 156     (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))
 157
 158 /* Test if there is an empty slot available on the front ring.
 159  * (This is only meaningful from the front. )
 160  */
 161 #define RING_FULL(_r)                                                   \
 162     (RING_FREE_REQUESTS(_r) == 0)
 163
 164 /* Test if there are outstanding messages to be processed on a ring. */
 165 #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
 166     ((_r)->sring->rsp_prod - (_r)->rsp_cons)
 167
 168 #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
 169     ({                                                                  \
 170         unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;      \
 171         unsigned int rsp = RING_SIZE(_r) -                              \
 172                            ((_r)->req_cons - (_r)->rsp_prod_pvt);       \
 173         req < rsp ? req : rsp;                                          \
 174     })
 175
 176 /* Direct access to individual ring elements, by index. */
 177 #define RING_GET_REQUEST(_r, _idx)                                      \
 178     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
 179
 180 #define RING_GET_RESPONSE(_r, _idx)                                     \
 181     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
 182
 183 /* Loop termination condition: Would the specified index overflow the ring? */
 184 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
 185     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
 186
 187 #define RING_PUSH_REQUESTS(_r) do {                                     \
 188     wmb(); /* back sees requests /before/ updated producer index */     \
 189     (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \
 190 } while (0)
 191
 192 #define RING_PUSH_RESPONSES(_r) do {                                    \
 193     wmb(); /* front sees responses /before/ updated producer index */   \
 194     (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
 195 } while (0)
 196
 197 /*
 198  * Notification hold-off (req_event and rsp_event):
 199  *
 200  * When queueing requests or responses on a shared ring, it may not always be
 201  * necessary to notify the remote end. For example, if requests are in flight
 202  * in a backend, the front may be able to queue further requests without
 203  * notifying the back (if the back checks for new requests when it queues
 204  * responses).
 205  *
 206  * When enqueuing requests or responses:
 207  *
 208  *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
 209  *  is a boolean return value. True indicates that the receiver requires an
 210  *  asynchronous notification.
 211  *
 212  * After dequeuing requests or responses (before sleeping the connection):
 213  *
 214  *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
 215  *  The second argument is a boolean return value. True indicates that there
 216  *  are pending messages on the ring (i.e., the connection should not be put
 217  *  to sleep).
 218  *
 219  *  These macros will set the req_event/rsp_event field to trigger a
 220  *  notification on the very next message that is enqueued. If you want to
 221  *  create batches of work (i.e., only receive a notification after several
 222  *  messages have been enqueued) then you will need to create a customised
 223  *  version of the FINAL_CHECK macro in your own code, which sets the event
 224  *  field appropriately.
 225  */
 226
 227 #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
 228     RING_IDX __old = (_r)->sring->req_prod;                             \
 229     RING_IDX __new = (_r)->req_prod_pvt;                                \
 230     wmb(); /* back sees requests /before/ updated producer index */     \
 231     (_r)->sring->req_prod = __new;                                      \
 232     mb(); /* back sees new requests /before/ we check req_event */      \
 233     (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
 234                  (RING_IDX)(__new - __old));                            \
 235 } while (0)
 236
 237 #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
 238     RING_IDX __old = (_r)->sring->rsp_prod;                             \
 239     RING_IDX __new = (_r)->rsp_prod_pvt;                                \
 240     wmb(); /* front sees responses /before/ updated producer index */   \
 241     (_r)->sring->rsp_prod = __new;                                      \
 242     mb(); /* front sees new responses /before/ we check rsp_event */    \
 243     (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
 244                  (RING_IDX)(__new - __old));                            \
 245 } while (0)
 246
 247 #define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
 248     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 249     if (_work_to_do) break;                                             \
 250     (_r)->sring->req_event = (_r)->req_cons + 1;                        \
 251     mb();                                                               \
 252     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 253 } while (0)
 254
 255 #define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
 256     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 257     if (_work_to_do) break;                                             \
 258     (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
 259     mb();                                                               \
 260     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 261 } while (0)
 262
 263 #endif /* __XEN_PUBLIC_IO_RING_H__ */
 264
 265 /*
 266  * Local variables:
 267  * mode: C
 268  * c-set-style: "BSD"
 269  * c-basic-offset: 4
 270  * tab-width: 4
 271  * indent-tabs-mode: nil
 272  * End:
 273  */