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 __CONST_RING_SIZE(_s, _sz)                              \
  28         (__RD32(((_sz) - offsetof(struct _s##_sring, ring)) /   \
  29                 sizeof(((struct _s##_sring *)0)->ring[0])))
  30
  31 /*
  32  * The same for passing in an actual pointer instead of a name tag.
  33  */
  34 #define __RING_SIZE(_s, _sz)                                            \
  35         (__RD32(((_sz) - (long)&(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))
  36
  37 /*
  38  * Macros to make the correct C datatypes for a new kind of ring.
  39  *
  40  * To make a new ring datatype, you need to have two message structures,
  41  * let's say struct request, and struct response already defined.
  42  *
  43  * In a header where you want the ring datatype declared, you then do:
  44  *
  45  *     DEFINE_RING_TYPES(mytag, struct request, struct response);
  46  *
  47  * These expand out to give you a set of types, as you can see below.
  48  * The most important of these are:
  49  *
  50  *     struct mytag_sring      - The shared ring.
  51  *     struct mytag_front_ring - The 'front' half of the ring.
  52  *     struct mytag_back_ring  - The 'back' half of the ring.
  53  *
  54  * To initialize a ring in your code you need to know the location and size
  55  * of the shared memory area (PAGE_SIZE, for instance). To initialise
  56  * the front half:
  57  *
  58  *     struct mytag_front_ring front_ring;
  59  *     SHARED_RING_INIT((struct mytag_sring *)shared_page);
  60  *     FRONT_RING_INIT(&front_ring, (struct mytag_sring *)shared_page,
  61  *                     PAGE_SIZE);
  62  *
  63  * Initializing the back follows similarly (note that only the front
  64  * initializes the shared ring):
  65  *
  66  *     struct mytag_back_ring back_ring;
  67  *     BACK_RING_INIT(&back_ring, (struct mytag_sring *)shared_page,
  68  *                    PAGE_SIZE);
  69  */
  70
  71 #define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
  72                                                                         \
  73 /* Shared ring entry */                                                 \
  74 union __name##_sring_entry {                                            \
  75     __req_t req;                                                        \
  76     __rsp_t rsp;                                                        \
  77 };                                                                      \
  78                                                                         \
  79 /* Shared ring page */                                                  \
  80 struct __name##_sring {                                                 \
  81     RING_IDX req_prod, req_event;                                       \
  82     RING_IDX rsp_prod, rsp_event;                                       \
  83     uint8_t  pad[48];                                                   \
  84     union __name##_sring_entry ring[1]; /* variable-length */           \
  85 };                                                                      \
  86                                                                         \
  87 /* "Front" end's private variables */                                   \
  88 struct __name##_front_ring {                                            \
  89     RING_IDX req_prod_pvt;                                              \
  90     RING_IDX rsp_cons;                                                  \
  91     unsigned int nr_ents;                                               \
  92     struct __name##_sring *sring;                                       \
  93 };                                                                      \
  94                                                                         \
  95 /* "Back" end's private variables */                                    \
  96 struct __name##_back_ring {                                             \
  97     RING_IDX rsp_prod_pvt;                                              \
  98     RING_IDX req_cons;                                                  \
  99     unsigned int nr_ents;                                               \
 100     struct __name##_sring *sring;                                       \
 101 };
 102
 103 /*
 104  * Macros for manipulating rings.
 105  *
 106  * FRONT_RING_whatever works on the "front end" of a ring: here
 107  * requests are pushed on to the ring and responses taken off it.
 108  *
 109  * BACK_RING_whatever works on the "back end" of a ring: here
 110  * requests are taken off the ring and responses put on.
 111  *
 112  * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL.
 113  * This is OK in 1-for-1 request-response situations where the
 114  * requestor (front end) never has more than RING_SIZE()-1
 115  * outstanding requests.
 116  */
 117
 118 /* Initialising empty rings */
 119 #define SHARED_RING_INIT(_s) do {                                       \
 120     (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
 121     (_s)->req_event = (_s)->rsp_event = 1;                              \
 122     memset((_s)->pad, 0, sizeof((_s)->pad));                            \
 123 } while(0)
 124
 125 #define FRONT_RING_INIT(_r, _s, __size) do {                            \
 126     (_r)->req_prod_pvt = 0;                                             \
 127     (_r)->rsp_cons = 0;                                                 \
 128     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 129     (_r)->sring = (_s);                                                 \
 130 } while (0)
 131
 132 #define BACK_RING_INIT(_r, _s, __size) do {                             \
 133     (_r)->rsp_prod_pvt = 0;                                             \
 134     (_r)->req_cons = 0;                                                 \
 135     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 136     (_r)->sring = (_s);                                                 \
 137 } while (0)
 138
 139 /* Initialize to existing shared indexes -- for recovery */
 140 #define FRONT_RING_ATTACH(_r, _s, __size) do {                          \
 141     (_r)->sring = (_s);                                                 \
 142     (_r)->req_prod_pvt = (_s)->req_prod;                                \
 143     (_r)->rsp_cons = (_s)->rsp_prod;                                    \
 144     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 145 } while (0)
 146
 147 #define BACK_RING_ATTACH(_r, _s, __size) do {                           \
 148     (_r)->sring = (_s);                                                 \
 149     (_r)->rsp_prod_pvt = (_s)->rsp_prod;                                \
 150     (_r)->req_cons = (_s)->req_prod;                                    \
 151     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 152 } while (0)
 153
 154 /* How big is this ring? */
 155 #define RING_SIZE(_r)                                                   \
 156     ((_r)->nr_ents)
 157
 158 /* Number of free requests (for use on front side only). */
 159 #define RING_FREE_REQUESTS(_r)                                          \
 160     (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))
 161
 162 /* Test if there is an empty slot available on the front ring.
 163  * (This is only meaningful from the front. )
 164  */
 165 #define RING_FULL(_r)                                                   \
 166     (RING_FREE_REQUESTS(_r) == 0)
 167
 168 /* Test if there are outstanding messages to be processed on a ring. */
 169 #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
 170     ((_r)->sring->rsp_prod - (_r)->rsp_cons)
 171
 172 #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
 173     ({                                                                  \
 174         unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;      \
 175         unsigned int rsp = RING_SIZE(_r) -                              \
 176                            ((_r)->req_cons - (_r)->rsp_prod_pvt);       \
 177         req < rsp ? req : rsp;                                          \
 178     })
 179
 180 /* Direct access to individual ring elements, by index. */
 181 #define RING_GET_REQUEST(_r, _idx)                                      \
 182     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
 183
 184 #define RING_GET_RESPONSE(_r, _idx)                                     \
 185     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
 186
 187 /* Loop termination condition: Would the specified index overflow the ring? */
 188 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
 189     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
 190
 191 #define RING_PUSH_REQUESTS(_r) do {                                     \
 192     wmb(); /* back sees requests /before/ updated producer index */     \
 193     (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \
 194 } while (0)
 195
 196 #define RING_PUSH_RESPONSES(_r) do {                                    \
 197     wmb(); /* front sees responses /before/ updated producer index */   \
 198     (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
 199 } while (0)
 200
 201 /*
 202  * Notification hold-off (req_event and rsp_event):
 203  *
 204  * When queueing requests or responses on a shared ring, it may not always be
 205  * necessary to notify the remote end. For example, if requests are in flight
 206  * in a backend, the front may be able to queue further requests without
 207  * notifying the back (if the back checks for new requests when it queues
 208  * responses).
 209  *
 210  * When enqueuing requests or responses:
 211  *
 212  *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
 213  *  is a boolean return value. True indicates that the receiver requires an
 214  *  asynchronous notification.
 215  *
 216  * After dequeuing requests or responses (before sleeping the connection):
 217  *
 218  *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
 219  *  The second argument is a boolean return value. True indicates that there
 220  *  are pending messages on the ring (i.e., the connection should not be put
 221  *  to sleep).
 222  *
 223  *  These macros will set the req_event/rsp_event field to trigger a
 224  *  notification on the very next message that is enqueued. If you want to
 225  *  create batches of work (i.e., only receive a notification after several
 226  *  messages have been enqueued) then you will need to create a customised
 227  *  version of the FINAL_CHECK macro in your own code, which sets the event
 228  *  field appropriately.
 229  */
 230
 231 #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
 232     RING_IDX __old = (_r)->sring->req_prod;                             \
 233     RING_IDX __new = (_r)->req_prod_pvt;                                \
 234     wmb(); /* back sees requests /before/ updated producer index */     \
 235     (_r)->sring->req_prod = __new;                                      \
 236     mb(); /* back sees new requests /before/ we check req_event */      \
 237     (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
 238                  (RING_IDX)(__new - __old));                            \
 239 } while (0)
 240
 241 #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
 242     RING_IDX __old = (_r)->sring->rsp_prod;                             \
 243     RING_IDX __new = (_r)->rsp_prod_pvt;                                \
 244     wmb(); /* front sees responses /before/ updated producer index */   \
 245     (_r)->sring->rsp_prod = __new;                                      \
 246     mb(); /* front sees new responses /before/ we check rsp_event */    \
 247     (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
 248                  (RING_IDX)(__new - __old));                            \
 249 } while (0)
 250
 251 #define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
 252     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 253     if (_work_to_do) break;                                             \
 254     (_r)->sring->req_event = (_r)->req_cons + 1;                        \
 255     mb();                                                               \
 256     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 257 } while (0)
 258
 259 #define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
 260     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 261     if (_work_to_do) break;                                             \
 262     (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
 263     mb();                                                               \
 264     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 265 } while (0)
 266
 267 #endif /* __XEN_PUBLIC_IO_RING_H__ */