1 /******************************************************************************
   2  * ring.h
   3  * 
   4  * Shared producer-consumer ring macros.
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to
   8  * deal in the Software without restriction, including without limitation the
   9  * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
  10  * sell copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  19  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  21  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  22  * DEALINGS IN THE SOFTWARE.
  23  *
  24  * Tim Deegan and Andrew Warfield November 2004.
  25  */
  26 
  27 #ifndef __XEN_PUBLIC_IO_RING_H__
  28 #define __XEN_PUBLIC_IO_RING_H__
  29 
  30 #include "../xen-compat.h"
  31 
  32 #if __XEN_INTERFACE_VERSION__ < 0x00030208
  33 #define xen_mb()  mb()
  34 #define xen_rmb() rmb()
  35 #define xen_wmb() wmb()
  36 #endif
  37 
  38 typedef unsigned int RING_IDX;
  39 
  40 /* Round a 32-bit unsigned constant down to the nearest power of two. */
  41 #define __RD2(_x)  (((_x) & 0x00000002) ? 0x2                  : ((_x) & 0x1))
  42 #define __RD4(_x)  (((_x) & 0x0000000c) ? __RD2((_x)>>2)<<2    : __RD2(_x))
  43 #define __RD8(_x)  (((_x) & 0x000000f0) ? __RD4((_x)>>4)<<4    : __RD4(_x))
  44 #define __RD16(_x) (((_x) & 0x0000ff00) ? __RD8((_x)>>8)<<8    : __RD8(_x))
  45 #define __RD32(_x) (((_x) & 0xffff0000) ? __RD16((_x)>>16)<<16 : __RD16(_x))
  46 
  47 /*
  48  * Calculate size of a shared ring, given the total available space for the
  49  * ring and indexes (_sz), and the name tag of the request/response structure.
  50  * A ring contains as many entries as will fit, rounded down to the nearest 
  51  * power of two (so we can mask with (size-1) to loop around).
  52  */
  53 #define __RING_SIZE(_s, _sz) \
  54     (__RD32(((_sz) - (long)(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))
  55 
  56 /*
  57  * Macros to make the correct C datatypes for a new kind of ring.
  58  * 
  59  * To make a new ring datatype, you need to have two message structures,
  60  * let's say request_t, and response_t already defined.
  61  *
  62  * In a header where you want the ring datatype declared, you then do:
  63  *
  64  *     DEFINE_RING_TYPES(mytag, request_t, response_t);
  65  *
  66  * These expand out to give you a set of types, as you can see below.
  67  * The most important of these are:
  68  * 
  69  *     mytag_sring_t      - The shared ring.
  70  *     mytag_front_ring_t - The 'front' half of the ring.
  71  *     mytag_back_ring_t  - The 'back' half of the ring.
  72  *
  73  * To initialize a ring in your code you need to know the location and size
  74  * of the shared memory area (PAGE_SIZE, for instance). To initialise
  75  * the front half:
  76  *
  77  *     mytag_front_ring_t front_ring;
  78  *     SHARED_RING_INIT((mytag_sring_t *)shared_page);
  79  *     FRONT_RING_INIT(&front_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
  80  *
  81  * Initializing the back follows similarly (note that only the front
  82  * initializes the shared ring):
  83  *
  84  *     mytag_back_ring_t back_ring;
  85  *     BACK_RING_INIT(&back_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
  86  */
  87 
  88 #define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
  89                                                                         \
  90 /* Shared ring entry */                                                 \
  91 union __name##_sring_entry {                                            \
  92     __req_t req;                                                        \
  93     __rsp_t rsp;                                                        \
  94 };                                                                      \
  95                                                                         \
  96 /* Shared ring page */                                                  \
  97 struct __name##_sring {                                                 \
  98     RING_IDX req_prod, req_event;                                       \
  99     RING_IDX rsp_prod, rsp_event;                                       \
 100     uint8_t  pad[48];                                                   \
 101     union __name##_sring_entry ring[1]; /* variable-length */           \
 102 };                                                                      \
 103                                                                         \
 104 /* "Front" end's private variables */                                   \
 105 struct __name##_front_ring {                                            \
 106     RING_IDX req_prod_pvt;                                              \
 107     RING_IDX rsp_cons;                                                  \
 108     unsigned int nr_ents;                                               \
 109     struct __name##_sring *sring;                                       \
 110 };                                                                      \
 111                                                                         \
 112 /* "Back" end's private variables */                                    \
 113 struct __name##_back_ring {                                             \
 114     RING_IDX rsp_prod_pvt;                                              \
 115     RING_IDX req_cons;                                                  \
 116     unsigned int nr_ents;                                               \
 117     struct __name##_sring *sring;                                       \
 118 };                                                                      \
 119                                                                         \
 120 /* Syntactic sugar */                                                   \
 121 typedef struct __name##_sring __name##_sring_t;                         \
 122 typedef struct __name##_front_ring __name##_front_ring_t;               \
 123 typedef struct __name##_back_ring __name##_back_ring_t
 124 
 125 /*
 126  * Macros for manipulating rings.
 127  * 
 128  * FRONT_RING_whatever works on the "front end" of a ring: here 
 129  * requests are pushed on to the ring and responses taken off it.
 130  * 
 131  * BACK_RING_whatever works on the "back end" of a ring: here 
 132  * requests are taken off the ring and responses put on.
 133  * 
 134  * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL. 
 135  * This is OK in 1-for-1 request-response situations where the 
 136  * requestor (front end) never has more than RING_SIZE()-1
 137  * outstanding requests.
 138  */
 139 
 140 /* Initialising empty rings */
 141 #define SHARED_RING_INIT(_s) do {                                       \
 142     (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
 143     (_s)->req_event = (_s)->rsp_event = 1;                              \
 144     (void)memset((_s)->pad, 0, sizeof((_s)->pad));                      \
 145 } while(0)
 146 
 147 #define FRONT_RING_INIT(_r, _s, __size) do {                            \
 148     (_r)->req_prod_pvt = 0;                                             \
 149     (_r)->rsp_cons = 0;                                                 \
 150     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 151     (_r)->sring = (_s);                                                 \
 152 } while (0)
 153 
 154 #define BACK_RING_INIT(_r, _s, __size) do {                             \
 155     (_r)->rsp_prod_pvt = 0;                                             \
 156     (_r)->req_cons = 0;                                                 \
 157     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 158     (_r)->sring = (_s);                                                 \
 159 } while (0)
 160 
 161 /* Initialize to existing shared indexes -- for recovery */
 162 #define FRONT_RING_ATTACH(_r, _s, __size) do {                          \
 163     (_r)->sring = (_s);                                                 \
 164     (_r)->req_prod_pvt = (_s)->req_prod;                                \
 165     (_r)->rsp_cons = (_s)->rsp_prod;                                    \
 166     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 167 } while (0)
 168 
 169 #define BACK_RING_ATTACH(_r, _s, __size) do {                           \
 170     (_r)->sring = (_s);                                                 \
 171     (_r)->rsp_prod_pvt = (_s)->rsp_prod;                                \
 172     (_r)->req_cons = (_s)->req_prod;                                    \
 173     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
 174 } while (0)
 175 
 176 /* How big is this ring? */
 177 #define RING_SIZE(_r)                                                   \
 178     ((_r)->nr_ents)
 179 
 180 /* Number of free requests (for use on front side only). */
 181 #define RING_FREE_REQUESTS(_r)                                          \
 182     (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))
 183 
 184 /* Test if there is an empty slot available on the front ring.
 185  * (This is only meaningful from the front. )
 186  */
 187 #define RING_FULL(_r)                                                   \
 188     (RING_FREE_REQUESTS(_r) == 0)
 189 
 190 /* Test if there are outstanding messages to be processed on a ring. */
 191 #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
 192     ((_r)->sring->rsp_prod - (_r)->rsp_cons)
 193 
 194 #ifdef __GNUC__
 195 #define RING_HAS_UNCONSUMED_REQUESTS(_r) ({                             \
 196     unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;          \
 197     unsigned int rsp = RING_SIZE(_r) -                                  \
 198         ((_r)->req_cons - (_r)->rsp_prod_pvt);                          \
 199     req < rsp ? req : rsp;                                              \
 200 })
 201 #else
 202 /* Same as above, but without the nice GCC ({ ... }) syntax. */
 203 #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
 204     ((((_r)->sring->req_prod - (_r)->req_cons) <                        \
 205       (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt))) ?        \
 206      ((_r)->sring->req_prod - (_r)->req_cons) :                         \
 207      (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt)))
 208 #endif
 209 
 210 /* Direct access to individual ring elements, by index. */
 211 #define RING_GET_REQUEST(_r, _idx)                                      \
 212     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
 213 
 214 #define RING_GET_RESPONSE(_r, _idx)                                     \
 215     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
 216 
 217 /* Loop termination condition: Would the specified index overflow the ring? */
 218 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
 219     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
 220 
 221 #define RING_PUSH_REQUESTS(_r) do {                                     \
 222     xen_wmb(); /* back sees requests /before/ updated producer index */ \
 223     (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \
 224 } while (0)
 225 
 226 #define RING_PUSH_RESPONSES(_r) do {                                    \
 227     xen_wmb(); /* front sees resps /before/ updated producer index */   \
 228     (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
 229 } while (0)
 230 
 231 /*
 232  * Notification hold-off (req_event and rsp_event):
 233  * 
 234  * When queueing requests or responses on a shared ring, it may not always be
 235  * necessary to notify the remote end. For example, if requests are in flight
 236  * in a backend, the front may be able to queue further requests without
 237  * notifying the back (if the back checks for new requests when it queues
 238  * responses).
 239  * 
 240  * When enqueuing requests or responses:
 241  * 
 242  *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
 243  *  is a boolean return value. True indicates that the receiver requires an
 244  *  asynchronous notification.
 245  * 
 246  * After dequeuing requests or responses (before sleeping the connection):
 247  * 
 248  *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
 249  *  The second argument is a boolean return value. True indicates that there
 250  *  are pending messages on the ring (i.e., the connection should not be put
 251  *  to sleep).
 252  * 
 253  *  These macros will set the req_event/rsp_event field to trigger a
 254  *  notification on the very next message that is enqueued. If you want to
 255  *  create batches of work (i.e., only receive a notification after several
 256  *  messages have been enqueued) then you will need to create a customised
 257  *  version of the FINAL_CHECK macro in your own code, which sets the event
 258  *  field appropriately.
 259  */
 260 
 261 #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
 262     RING_IDX __old = (_r)->sring->req_prod;                             \
 263     RING_IDX __new = (_r)->req_prod_pvt;                                \
 264     xen_wmb(); /* back sees requests /before/ updated producer index */ \
 265     (_r)->sring->req_prod = __new;                                      \
 266     xen_mb(); /* back sees new requests /before/ we check req_event */  \
 267     (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
 268                  (RING_IDX)(__new - __old));                            \
 269 } while (0)
 270 
 271 #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
 272     RING_IDX __old = (_r)->sring->rsp_prod;                             \
 273     RING_IDX __new = (_r)->rsp_prod_pvt;                                \
 274     xen_wmb(); /* front sees resps /before/ updated producer index */   \
 275     (_r)->sring->rsp_prod = __new;                                      \
 276     xen_mb(); /* front sees new resps /before/ we check rsp_event */    \
 277     (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
 278                  (RING_IDX)(__new - __old));                            \
 279 } while (0)
 280 
 281 #define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
 282     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 283     if (_work_to_do) break;                                             \
 284     (_r)->sring->req_event = (_r)->req_cons + 1;                        \
 285     xen_mb();                                                           \
 286     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
 287 } while (0)
 288 
 289 #define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
 290     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 291     if (_work_to_do) break;                                             \
 292     (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
 293     xen_mb();                                                           \
 294     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
 295 } while (0)
 296 
 297 #endif /* __XEN_PUBLIC_IO_RING_H__ */
 298 
 299 /*
 300  * Local variables:
 301  * mode: C
 302  * c-set-style: "BSD"
 303  * c-basic-offset: 4
 304  * tab-width: 4
 305  * indent-tabs-mode: nil
 306  * End:
 307  */