block/linux-aio.c

   1 /*
   2  * Linux native AIO support.
   3  *
   4  * Copyright (C) 2009 IBM, Corp.
   5  * Copyright (C) 2009 Red Hat, Inc.
   6  *
   7  * This work is licensed under the terms of the GNU GPL, version 2 or later.
   8  * See the COPYING file in the top-level directory.
   9  */
  10 #include "qemu/osdep.h"
  11 #include "block/aio.h"
  12 #include "qemu/queue.h"
  13 #include "block/block.h"
  14 #include "block/raw-aio.h"
  15 #include "qemu/event_notifier.h"
  16 #include "qemu/coroutine.h"
  17 #include "qapi/error.h"
  18
  19 #include <libaio.h>
  20
  21 /*
  22  * Queue size (per-device).
  23  *
  24  * XXX: eventually we need to communicate this to the guest and/or make it
  25  *      tunable by the guest.  If we get more outstanding requests at a time
  26  *      than this we will get EAGAIN from io_submit which is communicated to
  27  *      the guest as an I/O error.
  28  */
  29 #define MAX_EVENTS 1024
  30
  31 struct qemu_laiocb {
  32     Coroutine *co;
  33     LinuxAioState *ctx;
  34     struct iocb iocb;
  35     ssize_t ret;
  36     size_t nbytes;
  37     QEMUIOVector *qiov;
  38     bool is_read;
  39     QSIMPLEQ_ENTRY(qemu_laiocb) next;
  40 };
  41
  42 typedef struct {
  43     int plugged;
  44     unsigned int in_queue;
  45     unsigned int in_flight;
  46     bool blocked;
  47     QSIMPLEQ_HEAD(, qemu_laiocb) pending;
  48 } LaioQueue;
  49
  50 struct LinuxAioState {
  51     AioContext *aio_context;
  52
  53     io_context_t ctx;
  54     EventNotifier e;
  55
  56     /* io queue for submit at batch.  Protected by AioContext lock. */
  57     LaioQueue io_q;
  58
  59     /* I/O completion processing.  Only runs in I/O thread.  */
  60     QEMUBH *completion_bh;
  61     int event_idx;
  62     int event_max;
  63 };
  64
  65 static void ioq_submit(LinuxAioState *s);
  66
  67 static inline ssize_t io_event_ret(struct io_event *ev)
  68 {
  69     return (ssize_t)(((uint64_t)ev->res2 << 32) | ev->res);
  70 }
  71
  72 /*
  73  * Completes an AIO request.
  74  */
  75 static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
  76 {
  77     int ret;
  78
  79     ret = laiocb->ret;
  80     if (ret != -ECANCELED) {
  81         if (ret == laiocb->nbytes) {
  82             ret = 0;
  83         } else if (ret >= 0) {
  84             /* Short reads mean EOF, pad with zeros. */
  85             if (laiocb->is_read) {
  86                 qemu_iovec_memset(laiocb->qiov, ret, 0,
  87                     laiocb->qiov->size - ret);
  88             } else {
  89                 ret = -ENOSPC;
  90             }
  91         }
  92     }
  93
  94     laiocb->ret = ret;
  95
  96     /*
  97      * If the coroutine is already entered it must be in ioq_submit() and
  98      * will notice laio->ret has been filled in when it eventually runs
  99      * later.  Coroutines cannot be entered recursively so avoid doing
 100      * that!
 101      */
 102     if (!qemu_coroutine_entered(laiocb->co)) {
 103         aio_co_wake(laiocb->co);
 104     }
 105 }
 106
 107 /**
 108  * aio_ring buffer which is shared between userspace and kernel.
 109  *
 110  * This copied from linux/fs/aio.c, common header does not exist
 111  * but AIO exists for ages so we assume ABI is stable.
 112  */
 113 struct aio_ring {
 114     unsigned    id;    /* kernel internal index number */
 115     unsigned    nr;    /* number of io_events */
 116     unsigned    head;  /* Written to by userland or by kernel. */
 117     unsigned    tail;
 118
 119     unsigned    magic;
 120     unsigned    compat_features;
 121     unsigned    incompat_features;
 122     unsigned    header_length;  /* size of aio_ring */
 123
 124     struct io_event io_events[];
 125 };
 126
 127 /**
 128  * io_getevents_peek:
 129  * @ctx: AIO context
 130  * @events: pointer on events array, output value
 131
 132  * Returns the number of completed events and sets a pointer
 133  * on events array.  This function does not update the internal
 134  * ring buffer, only reads head and tail.  When @events has been
 135  * processed io_getevents_commit() must be called.
 136  */
 137 static inline unsigned int io_getevents_peek(io_context_t ctx,
 138                                              struct io_event **events)
 139 {
 140     struct aio_ring *ring = (struct aio_ring *)ctx;
 141     unsigned int head = ring->head, tail = ring->tail;
 142     unsigned int nr;
 143
 144     nr = tail >= head ? tail - head : ring->nr - head;
 145     *events = ring->io_events + head;
 146     /* To avoid speculative loads of s->events[i] before observing tail.
 147        Paired with smp_wmb() inside linux/fs/aio.c: aio_complete(). */
 148     smp_rmb();
 149
 150     return nr;
 151 }
 152
 153 /**
 154  * io_getevents_commit:
 155  * @ctx: AIO context
 156  * @nr: the number of events on which head should be advanced
 157  *
 158  * Advances head of a ring buffer.
 159  */
 160 static inline void io_getevents_commit(io_context_t ctx, unsigned int nr)
 161 {
 162     struct aio_ring *ring = (struct aio_ring *)ctx;
 163
 164     if (nr) {
 165         ring->head = (ring->head + nr) % ring->nr;
 166     }
 167 }
 168
 169 /**
 170  * io_getevents_advance_and_peek:
 171  * @ctx: AIO context
 172  * @events: pointer on events array, output value
 173  * @nr: the number of events on which head should be advanced
 174  *
 175  * Advances head of a ring buffer and returns number of elements left.
 176  */
 177 static inline unsigned int
 178 io_getevents_advance_and_peek(io_context_t ctx,
 179                               struct io_event **events,
 180                               unsigned int nr)
 181 {
 182     io_getevents_commit(ctx, nr);
 183     return io_getevents_peek(ctx, events);
 184 }
 185
 186 /**
 187  * qemu_laio_process_completions:
 188  * @s: AIO state
 189  *
 190  * Fetches completed I/O requests and invokes their callbacks.
 191  *
 192  * The function is somewhat tricky because it supports nested event loops, for
 193  * example when a request callback invokes aio_poll().  In order to do this,
 194  * indices are kept in LinuxAioState.  Function schedules BH completion so it
 195  * can be called again in a nested event loop.  When there are no events left
 196  * to complete the BH is being canceled.
 197  */
 198 static void qemu_laio_process_completions(LinuxAioState *s)
 199 {
 200     struct io_event *events;
 201
 202     /* Reschedule so nested event loops see currently pending completions */
 203     qemu_bh_schedule(s->completion_bh);
 204
 205     while ((s->event_max = io_getevents_advance_and_peek(s->ctx, &events,
 206                                                          s->event_idx))) {
 207         for (s->event_idx = 0; s->event_idx < s->event_max; ) {
 208             struct iocb *iocb = events[s->event_idx].obj;
 209             struct qemu_laiocb *laiocb =
 210                 container_of(iocb, struct qemu_laiocb, iocb);
 211
 212             laiocb->ret = io_event_ret(&events[s->event_idx]);
 213
 214             /* Change counters one-by-one because we can be nested. */
 215             s->io_q.in_flight--;
 216             s->event_idx++;
 217             qemu_laio_process_completion(laiocb);
 218         }
 219     }
 220
 221     qemu_bh_cancel(s->completion_bh);
 222
 223     /* If we are nested we have to notify the level above that we are done
 224      * by setting event_max to zero, upper level will then jump out of it's
 225      * own `for` loop.  If we are the last all counters droped to zero. */
 226     s->event_max = 0;
 227     s->event_idx = 0;
 228 }
 229
 230 static void qemu_laio_process_completions_and_submit(LinuxAioState *s)
 231 {
 232     aio_context_acquire(s->aio_context);
 233     qemu_laio_process_completions(s);
 234
 235     if (!s->io_q.plugged && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
 236         ioq_submit(s);
 237     }
 238     aio_context_release(s->aio_context);
 239 }
 240
 241 static void qemu_laio_completion_bh(void *opaque)
 242 {
 243     LinuxAioState *s = opaque;
 244
 245     qemu_laio_process_completions_and_submit(s);
 246 }
 247
 248 static void qemu_laio_completion_cb(EventNotifier *e)
 249 {
 250     LinuxAioState *s = container_of(e, LinuxAioState, e);
 251
 252     if (event_notifier_test_and_clear(&s->e)) {
 253         qemu_laio_process_completions_and_submit(s);
 254     }
 255 }
 256
 257 static bool qemu_laio_poll_cb(void *opaque)
 258 {
 259     EventNotifier *e = opaque;
 260     LinuxAioState *s = container_of(e, LinuxAioState, e);
 261     struct io_event *events;
 262
 263     if (!io_getevents_peek(s->ctx, &events)) {
 264         return false;
 265     }
 266
 267     qemu_laio_process_completions_and_submit(s);
 268     return true;
 269 }
 270
 271 static void ioq_init(LaioQueue *io_q)
 272 {
 273     QSIMPLEQ_INIT(&io_q->pending);
 274     io_q->plugged = 0;
 275     io_q->in_queue = 0;
 276     io_q->in_flight = 0;
 277     io_q->blocked = false;
 278 }
 279
 280 static void ioq_submit(LinuxAioState *s)
 281 {
 282     int ret, len;
 283     struct qemu_laiocb *aiocb;
 284     struct iocb *iocbs[MAX_EVENTS];
 285     QSIMPLEQ_HEAD(, qemu_laiocb) completed;
 286
 287     do {
 288         if (s->io_q.in_flight >= MAX_EVENTS) {
 289             break;
 290         }
 291         len = 0;
 292         QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
 293             iocbs[len++] = &aiocb->iocb;
 294             if (s->io_q.in_flight + len >= MAX_EVENTS) {
 295                 break;
 296             }
 297         }
 298
 299         ret = io_submit(s->ctx, len, iocbs);
 300         if (ret == -EAGAIN) {
 301             break;
 302         }
 303         if (ret < 0) {
 304             /* Fail the first request, retry the rest */
 305             aiocb = QSIMPLEQ_FIRST(&s->io_q.pending);
 306             QSIMPLEQ_REMOVE_HEAD(&s->io_q.pending, next);
 307             s->io_q.in_queue--;
 308             aiocb->ret = ret;
 309             qemu_laio_process_completion(aiocb);
 310             continue;
 311         }
 312
 313         s->io_q.in_flight += ret;
 314         s->io_q.in_queue  -= ret;
 315         aiocb = container_of(iocbs[ret - 1], struct qemu_laiocb, iocb);
 316         QSIMPLEQ_SPLIT_AFTER(&s->io_q.pending, aiocb, next, &completed);
 317     } while (ret == len && !QSIMPLEQ_EMPTY(&s->io_q.pending));
 318     s->io_q.blocked = (s->io_q.in_queue > 0);
 319
 320     if (s->io_q.in_flight) {
 321         /* We can try to complete something just right away if there are
 322          * still requests in-flight. */
 323         qemu_laio_process_completions(s);
 324         /*
 325          * Even we have completed everything (in_flight == 0), the queue can
 326          * have still pended requests (in_queue > 0).  We do not attempt to
 327          * repeat submission to avoid IO hang.  The reason is simple: s->e is
 328          * still set and completion callback will be called shortly and all
 329          * pended requests will be submitted from there.
 330          */
 331     }
 332 }
 333
 334 void laio_io_plug(BlockDriverState *bs, LinuxAioState *s)
 335 {
 336     s->io_q.plugged++;
 337 }
 338
 339 void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s)
 340 {
 341     assert(s->io_q.plugged);
 342     if (--s->io_q.plugged == 0 &&
 343         !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
 344         ioq_submit(s);
 345     }
 346 }
 347
 348 static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
 349                           int type)
 350 {
 351     LinuxAioState *s = laiocb->ctx;
 352     struct iocb *iocbs = &laiocb->iocb;
 353     QEMUIOVector *qiov = laiocb->qiov;
 354
 355     switch (type) {
 356     case QEMU_AIO_WRITE:
 357         io_prep_pwritev(iocbs, fd, qiov->iov, qiov->niov, offset);
 358         break;
 359     case QEMU_AIO_READ:
 360         io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
 361         break;
 362     /* Currently Linux kernel does not support other operations */
 363     default:
 364         fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
 365                         __func__, type);
 366         return -EIO;
 367     }
 368     io_set_eventfd(&laiocb->iocb, event_notifier_get_fd(&s->e));
 369
 370     QSIMPLEQ_INSERT_TAIL(&s->io_q.pending, laiocb, next);
 371     s->io_q.in_queue++;
 372     if (!s->io_q.blocked &&
 373         (!s->io_q.plugged ||
 374          s->io_q.in_flight + s->io_q.in_queue >= MAX_EVENTS)) {
 375         ioq_submit(s);
 376     }
 377
 378     return 0;
 379 }
 380
 381 int coroutine_fn laio_co_submit(BlockDriverState *bs, LinuxAioState *s, int fd,
 382                                 uint64_t offset, QEMUIOVector *qiov, int type)
 383 {
 384     int ret;
 385     struct qemu_laiocb laiocb = {
 386         .co         = qemu_coroutine_self(),
 387         .nbytes     = qiov->size,
 388         .ctx        = s,
 389         .ret        = -EINPROGRESS,
 390         .is_read    = (type == QEMU_AIO_READ),
 391         .qiov       = qiov,
 392     };
 393
 394     ret = laio_do_submit(fd, &laiocb, offset, type);
 395     if (ret < 0) {
 396         return ret;
 397     }
 398
 399     if (laiocb.ret == -EINPROGRESS) {
 400         qemu_coroutine_yield();
 401     }
 402     return laiocb.ret;
 403 }
 404
 405 void laio_detach_aio_context(LinuxAioState *s, AioContext *old_context)
 406 {
 407     aio_set_event_notifier(old_context, &s->e, false, NULL, NULL);
 408     qemu_bh_delete(s->completion_bh);
 409     s->aio_context = NULL;
 410 }
 411
 412 void laio_attach_aio_context(LinuxAioState *s, AioContext *new_context)
 413 {
 414     s->aio_context = new_context;
 415     s->completion_bh = aio_bh_new(new_context, qemu_laio_completion_bh, s);
 416     aio_set_event_notifier(new_context, &s->e, false,
 417                            qemu_laio_completion_cb,
 418                            qemu_laio_poll_cb);
 419 }
 420
 421 LinuxAioState *laio_init(Error **errp)
 422 {
 423     int rc;
 424     LinuxAioState *s;
 425
 426     s = g_malloc0(sizeof(*s));
 427     rc = event_notifier_init(&s->e, false);
 428     if (rc < 0) {
 429         error_setg_errno(errp, -rc, "failed to to initialize event notifier");
 430         goto out_free_state;
 431     }
 432
 433     rc = io_setup(MAX_EVENTS, &s->ctx);
 434     if (rc < 0) {
 435         error_setg_errno(errp, -rc, "failed to create linux AIO context");
 436         goto out_close_efd;
 437     }
 438
 439     ioq_init(&s->io_q);
 440
 441     return s;
 442
 443 out_close_efd:
 444     event_notifier_cleanup(&s->e);
 445 out_free_state:
 446     g_free(s);
 447     return NULL;
 448 }
 449
 450 void laio_cleanup(LinuxAioState *s)
 451 {
 452     event_notifier_cleanup(&s->e);
 453
 454     if (io_destroy(s->ctx) != 0) {
 455         fprintf(stderr, "%s: destroy AIO context %p failed\n",
 456                         __func__, &s->ctx);
 457     }
 458     g_free(s);
 459 }