ring.h

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    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 __CONST_RING_SIZE(_s, _sz) \
   54     (__RD32(((_sz) - offsetof(struct _s##_sring, ring)) / \
   55             sizeof(((struct _s##_sring *)0)->ring[0])))
   56 /*
   57  * The same for passing in an actual pointer instead of a name tag.
   58  */
   59 #define __RING_SIZE(_s, _sz) \
   60     (__RD32(((_sz) - (long)(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))
   61 
   62 /*
   63  * Macros to make the correct C datatypes for a new kind of ring.
   64  * 
   65  * To make a new ring datatype, you need to have two message structures,
   66  * let's say request_t, and response_t already defined.
   67  *
   68  * In a header where you want the ring datatype declared, you then do:
   69  *
   70  *     DEFINE_RING_TYPES(mytag, request_t, response_t);
   71  *
   72  * These expand out to give you a set of types, as you can see below.
   73  * The most important of these are:
   74  * 
   75  *     mytag_sring_t      - The shared ring.
   76  *     mytag_front_ring_t - The 'front' half of the ring.
   77  *     mytag_back_ring_t  - The 'back' half of the ring.
   78  *
   79  * To initialize a ring in your code you need to know the location and size
   80  * of the shared memory area (PAGE_SIZE, for instance). To initialise
   81  * the front half:
   82  *
   83  *     mytag_front_ring_t front_ring;
   84  *     SHARED_RING_INIT((mytag_sring_t *)shared_page);
   85  *     FRONT_RING_INIT(&front_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
   86  *
   87  * Initializing the back follows similarly (note that only the front
   88  * initializes the shared ring):
   89  *
   90  *     mytag_back_ring_t back_ring;
   91  *     BACK_RING_INIT(&back_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
   92  */
   93 
   94 #define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
   95                                                                         \
   96 /* Shared ring entry */                                                 \
   97 union __name##_sring_entry {                                            \
   98     __req_t req;                                                        \
   99     __rsp_t rsp;                                                        \
  100 };                                                                      \
  101                                                                         \
  102 /* Shared ring page */                                                  \
  103 struct __name##_sring {                                                 \
  104     RING_IDX req_prod, req_event;                                       \
  105     RING_IDX rsp_prod, rsp_event;                                       \
  106     union {                                                             \
  107         struct {                                                        \
  108             uint8_t smartpoll_active;                                   \
  109         } netif;                                                        \
  110         struct {                                                        \
  111             uint8_t msg;                                                \
  112         } tapif_user;                                                   \
  113         uint8_t pvt_pad[4];                                             \
  114     } pvt;                                                              \
  115     uint8_t __pad[44];                                                  \
  116     union __name##_sring_entry ring[1]; /* variable-length */           \
  117 };                                                                      \
  118                                                                         \
  119 /* "Front" end's private variables */                                   \
  120 struct __name##_front_ring {                                            \
  121     RING_IDX req_prod_pvt;                                              \
  122     RING_IDX rsp_cons;                                                  \
  123     unsigned int nr_ents;                                               \
  124     struct __name##_sring *sring;                                       \
  125 };                                                                      \
  126                                                                         \
  127 /* "Back" end's private variables */                                    \
  128 struct __name##_back_ring {                                             \
  129     RING_IDX rsp_prod_pvt;                                              \
  130     RING_IDX req_cons;                                                  \
  131     unsigned int nr_ents;                                               \
  132     struct __name##_sring *sring;                                       \
  133 };                                                                      \
  134                                                                         \
  135 /* Syntactic sugar */                                                   \
  136 typedef struct __name##_sring __name##_sring_t;                         \
  137 typedef struct __name##_front_ring __name##_front_ring_t;               \
  138 typedef struct __name##_back_ring __name##_back_ring_t
  139 
  140 /*
  141  * Macros for manipulating rings.
  142  * 
  143  * FRONT_RING_whatever works on the "front end" of a ring: here 
  144  * requests are pushed on to the ring and responses taken off it.
  145  * 
  146  * BACK_RING_whatever works on the "back end" of a ring: here 
  147  * requests are taken off the ring and responses put on.
  148  * 
  149  * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL. 
  150  * This is OK in 1-for-1 request-response situations where the 
  151  * requestor (front end) never has more than RING_SIZE()-1
  152  * outstanding requests.
  153  */
  154 
  155 /* Initialising empty rings */
  156 #define SHARED_RING_INIT(_s) do {                                       \
  157     (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
  158     (_s)->req_event = (_s)->rsp_event = 1;                              \
  159     (void)memset((_s)->pvt.pvt_pad, 0, sizeof((_s)->pvt.pvt_pad));      \
  160     (void)memset((_s)->__pad, 0, sizeof((_s)->__pad));                  \
  161 } while(0)
  162 
  163 #define FRONT_RING_INIT(_r, _s, __size) do {                            \
  164     (_r)->req_prod_pvt = 0;                                             \
  165     (_r)->rsp_cons = 0;                                                 \
  166     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
  167     (_r)->sring = (_s);                                                 \
  168 } while (0)
  169 
  170 #define BACK_RING_INIT(_r, _s, __size) do {                             \
  171     (_r)->rsp_prod_pvt = 0;                                             \
  172     (_r)->req_cons = 0;                                                 \
  173     (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
  174     (_r)->sring = (_s);                                                 \
  175 } while (0)
  176 
  177 /* How big is this ring? */
  178 #define RING_SIZE(_r)                                                   \
  179     ((_r)->nr_ents)
  180 
  181 /* Number of free requests (for use on front side only). */
  182 #define RING_FREE_REQUESTS(_r)                                          \
  183     (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))
  184 
  185 /* Test if there is an empty slot available on the front ring.
  186  * (This is only meaningful from the front. )
  187  */
  188 #define RING_FULL(_r)                                                   \
  189     (RING_FREE_REQUESTS(_r) == 0)
  190 
  191 /* Test if there are outstanding messages to be processed on a ring. */
  192 #define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
  193     ((_r)->sring->rsp_prod - (_r)->rsp_cons)
  194 
  195 #ifdef __GNUC__
  196 #define RING_HAS_UNCONSUMED_REQUESTS(_r) ({                             \
  197     unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;          \
  198     unsigned int rsp = RING_SIZE(_r) -                                  \
  199         ((_r)->req_cons - (_r)->rsp_prod_pvt);                          \
  200     req < rsp ? req : rsp;                                              \
  201 })
  202 #else
  203 /* Same as above, but without the nice GCC ({ ... }) syntax. */
  204 #define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
  205     ((((_r)->sring->req_prod - (_r)->req_cons) <                        \
  206       (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt))) ?        \
  207      ((_r)->sring->req_prod - (_r)->req_cons) :                         \
  208      (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt)))
  209 #endif
  210 
  211 /* Direct access to individual ring elements, by index. */
  212 #define RING_GET_REQUEST(_r, _idx)                                      \
  213     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
  214 
  215 #define RING_GET_RESPONSE(_r, _idx)                                     \
  216     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
  217 
  218 /* Loop termination condition: Would the specified index overflow the ring? */
  219 #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
  220     (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
  221 
  222 /* Ill-behaved frontend determination: Can there be this many requests? */
  223 #define RING_REQUEST_PROD_OVERFLOW(_r, _prod)                           \
  224     (((_prod) - (_r)->rsp_prod_pvt) > RING_SIZE(_r))
  225 
  226 #define RING_PUSH_REQUESTS(_r) do {                                     \
  227     xen_wmb(); /* back sees requests /before/ updated producer index */ \
  228     (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \
  229 } while (0)
  230 
  231 #define RING_PUSH_RESPONSES(_r) do {                                    \
  232     xen_wmb(); /* front sees resps /before/ updated producer index */   \
  233     (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
  234 } while (0)
  235 
  236 /*
  237  * Notification hold-off (req_event and rsp_event):
  238  * 
  239  * When queueing requests or responses on a shared ring, it may not always be
  240  * necessary to notify the remote end. For example, if requests are in flight
  241  * in a backend, the front may be able to queue further requests without
  242  * notifying the back (if the back checks for new requests when it queues
  243  * responses).
  244  * 
  245  * When enqueuing requests or responses:
  246  * 
  247  *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
  248  *  is a boolean return value. True indicates that the receiver requires an
  249  *  asynchronous notification.
  250  * 
  251  * After dequeuing requests or responses (before sleeping the connection):
  252  * 
  253  *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
  254  *  The second argument is a boolean return value. True indicates that there
  255  *  are pending messages on the ring (i.e., the connection should not be put
  256  *  to sleep).
  257  * 
  258  *  These macros will set the req_event/rsp_event field to trigger a
  259  *  notification on the very next message that is enqueued. If you want to
  260  *  create batches of work (i.e., only receive a notification after several
  261  *  messages have been enqueued) then you will need to create a customised
  262  *  version of the FINAL_CHECK macro in your own code, which sets the event
  263  *  field appropriately.
  264  */
  265 
  266 #define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
  267     RING_IDX __old = (_r)->sring->req_prod;                             \
  268     RING_IDX __new = (_r)->req_prod_pvt;                                \
  269     xen_wmb(); /* back sees requests /before/ updated producer index */ \
  270     (_r)->sring->req_prod = __new;                                      \
  271     xen_mb(); /* back sees new requests /before/ we check req_event */  \
  272     (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
  273                  (RING_IDX)(__new - __old));                            \
  274 } while (0)
  275 
  276 #define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
  277     RING_IDX __old = (_r)->sring->rsp_prod;                             \
  278     RING_IDX __new = (_r)->rsp_prod_pvt;                                \
  279     xen_wmb(); /* front sees resps /before/ updated producer index */   \
  280     (_r)->sring->rsp_prod = __new;                                      \
  281     xen_mb(); /* front sees new resps /before/ we check rsp_event */    \
  282     (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
  283                  (RING_IDX)(__new - __old));                            \
  284 } while (0)
  285 
  286 #define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
  287     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
  288     if (_work_to_do) break;                                             \
  289     (_r)->sring->req_event = (_r)->req_cons + 1;                        \
  290     xen_mb();                                                           \
  291     (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
  292 } while (0)
  293 
  294 #define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
  295     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
  296     if (_work_to_do) break;                                             \
  297     (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
  298     xen_mb();                                                           \
  299     (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
  300 } while (0)
  301 
  302 #endif /* __XEN_PUBLIC_IO_RING_H__ */
  303 
  304 /*
  305  * Local variables:
  306  * mode: C
  307  * c-file-style: "BSD"
  308  * c-basic-offset: 4
  309  * tab-width: 4
  310  * indent-tabs-mode: nil
  311  * End:
  312  */
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FreeBSD/Linux Kernel Cross Reference sys/xen/interface/io/ring.h

FreeBSD/Linux Kernel Cross Reference
sys/xen/interface/io/ring.h
