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 /* Maximum number of requests in a batch. (default value) */
  32 #define DEFAULT_MAX_BATCH 32
  33
  34 struct qemu_laiocb {
  35     Coroutine *co;
  36     LinuxAioState *ctx;
  37     struct iocb iocb;
  38     ssize_t ret;
  39     size_t nbytes;
  40     QEMUIOVector *qiov;
  41     bool is_read;
  42     QSIMPLEQ_ENTRY(qemu_laiocb) next;
  43 };
  44
  45 typedef struct {
  46     int plugged;
  47     unsigned int in_queue;
  48     unsigned int in_flight;
  49     bool blocked;
  50     QSIMPLEQ_HEAD(, qemu_laiocb) pending;
  51 } LaioQueue;
  52
  53 struct LinuxAioState {
  54     AioContext *aio_context;
  55
  56     io_context_t ctx;
  57     EventNotifier e;
  58
  59     /* io queue for submit at batch.  Protected by AioContext lock. */
  60     LaioQueue io_q;
  61
  62     /* I/O completion processing.  Only runs in I/O thread.  */
  63     QEMUBH *completion_bh;
  64     int event_idx;
  65     int event_max;
  66 };
  67
  68 static void ioq_submit(LinuxAioState *s);
  69
  70 static inline ssize_t io_event_ret(struct io_event *ev)
  71 {
  72     return (ssize_t)(((uint64_t)ev->res2 << 32) | ev->res);
  73 }
  74
  75 /*
  76  * Completes an AIO request.
  77  */
  78 static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
  79 {
  80     int ret;
  81
  82     ret = laiocb->ret;
  83     if (ret != -ECANCELED) {
  84         if (ret == laiocb->nbytes) {
  85             ret = 0;
  86         } else if (ret >= 0) {
  87             /* Short reads mean EOF, pad with zeros. */
  88             if (laiocb->is_read) {
  89                 qemu_iovec_memset(laiocb->qiov, ret, 0,
  90                     laiocb->qiov->size - ret);
  91             } else {
  92                 ret = -ENOSPC;
  93             }
  94         }
  95     }
  96
  97     laiocb->ret = ret;
  98
  99     /*
 100      * If the coroutine is already entered it must be in ioq_submit() and
 101      * will notice laio->ret has been filled in when it eventually runs
 102      * later.  Coroutines cannot be entered recursively so avoid doing
 103      * that!
 104      */
 105     if (!qemu_coroutine_entered(laiocb->co)) {
 106         aio_co_wake(laiocb->co);
 107     }
 108 }
 109
 110 /**
 111  * aio_ring buffer which is shared between userspace and kernel.
 112  *
 113  * This copied from linux/fs/aio.c, common header does not exist
 114  * but AIO exists for ages so we assume ABI is stable.
 115  */
 116 struct aio_ring {
 117     unsigned    id;    /* kernel internal index number */
 118     unsigned    nr;    /* number of io_events */
 119     unsigned    head;  /* Written to by userland or by kernel. */
 120     unsigned    tail;
 121
 122     unsigned    magic;
 123     unsigned    compat_features;
 124     unsigned    incompat_features;
 125     unsigned    header_length;  /* size of aio_ring */
 126
 127     struct io_event io_events[];
 128 };
 129
 130 /**
 131  * io_getevents_peek:
 132  * @ctx: AIO context
 133  * @events: pointer on events array, output value
 134
 135  * Returns the number of completed events and sets a pointer
 136  * on events array.  This function does not update the internal
 137  * ring buffer, only reads head and tail.  When @events has been
 138  * processed io_getevents_commit() must be called.
 139  */
 140 static inline unsigned int io_getevents_peek(io_context_t ctx,
 141                                              struct io_event **events)
 142 {
 143     struct aio_ring *ring = (struct aio_ring *)ctx;
 144     unsigned int head = ring->head, tail = ring->tail;
 145     unsigned int nr;
 146
 147     nr = tail >= head ? tail - head : ring->nr - head;
 148     *events = ring->io_events + head;
 149     /* To avoid speculative loads of s->events[i] before observing tail.
 150        Paired with smp_wmb() inside linux/fs/aio.c: aio_complete(). */
 151     smp_rmb();
 152
 153     return nr;
 154 }
 155
 156 /**
 157  * io_getevents_commit:
 158  * @ctx: AIO context
 159  * @nr: the number of events on which head should be advanced
 160  *
 161  * Advances head of a ring buffer.
 162  */
 163 static inline void io_getevents_commit(io_context_t ctx, unsigned int nr)
 164 {
 165     struct aio_ring *ring = (struct aio_ring *)ctx;
 166
 167     if (nr) {
 168         ring->head = (ring->head + nr) % ring->nr;
 169     }
 170 }
 171
 172 /**
 173  * io_getevents_advance_and_peek:
 174  * @ctx: AIO context
 175  * @events: pointer on events array, output value
 176  * @nr: the number of events on which head should be advanced
 177  *
 178  * Advances head of a ring buffer and returns number of elements left.
 179  */
 180 static inline unsigned int
 181 io_getevents_advance_and_peek(io_context_t ctx,
 182                               struct io_event **events,
 183                               unsigned int nr)
 184 {
 185     io_getevents_commit(ctx, nr);
 186     return io_getevents_peek(ctx, events);
 187 }
 188
 189 /**
 190  * qemu_laio_process_completions:
 191  * @s: AIO state
 192  *
 193  * Fetches completed I/O requests and invokes their callbacks.
 194  *
 195  * The function is somewhat tricky because it supports nested event loops, for
 196  * example when a request callback invokes aio_poll().  In order to do this,
 197  * indices are kept in LinuxAioState.  Function schedules BH completion so it
 198  * can be called again in a nested event loop.  When there are no events left
 199  * to complete the BH is being canceled.
 200  */
 201 static void qemu_laio_process_completions(LinuxAioState *s)
 202 {
 203     struct io_event *events;
 204
 205     /* Reschedule so nested event loops see currently pending completions */
 206     qemu_bh_schedule(s->completion_bh);
 207
 208     while ((s->event_max = io_getevents_advance_and_peek(s->ctx, &events,
 209                                                          s->event_idx))) {
 210         for (s->event_idx = 0; s->event_idx < s->event_max; ) {
 211             struct iocb *iocb = events[s->event_idx].obj;
 212             struct qemu_laiocb *laiocb =
 213                 container_of(iocb, struct qemu_laiocb, iocb);
 214
 215             laiocb->ret = io_event_ret(&events[s->event_idx]);
 216
 217             /* Change counters one-by-one because we can be nested. */
 218             s->io_q.in_flight--;
 219             s->event_idx++;
 220             qemu_laio_process_completion(laiocb);
 221         }
 222     }
 223
 224     qemu_bh_cancel(s->completion_bh);
 225
 226     /* If we are nested we have to notify the level above that we are done
 227      * by setting event_max to zero, upper level will then jump out of it's
 228      * own `for` loop.  If we are the last all counters droped to zero. */
 229     s->event_max = 0;
 230     s->event_idx = 0;
 231 }
 232
 233 static void qemu_laio_process_completions_and_submit(LinuxAioState *s)
 234 {
 235     aio_context_acquire(s->aio_context);
 236     qemu_laio_process_completions(s);
 237
 238     if (!s->io_q.plugged && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
 239         ioq_submit(s);
 240     }
 241     aio_context_release(s->aio_context);
 242 }
 243
 244 static void qemu_laio_completion_bh(void *opaque)
 245 {
 246     LinuxAioState *s = opaque;
 247
 248     qemu_laio_process_completions_and_submit(s);
 249 }
 250
 251 static void qemu_laio_completion_cb(EventNotifier *e)
 252 {
 253     LinuxAioState *s = container_of(e, LinuxAioState, e);
 254
 255     if (event_notifier_test_and_clear(&s->e)) {
 256         qemu_laio_process_completions_and_submit(s);
 257     }
 258 }
 259
 260 static bool qemu_laio_poll_cb(void *opaque)
 261 {
 262     EventNotifier *e = opaque;
 263     LinuxAioState *s = container_of(e, LinuxAioState, e);
 264     struct io_event *events;
 265
 266     return io_getevents_peek(s->ctx, &events);
 267 }
 268
 269 static void qemu_laio_poll_ready(EventNotifier *opaque)
 270 {
 271     EventNotifier *e = opaque;
 272     LinuxAioState *s = container_of(e, LinuxAioState, e);
 273
 274     qemu_laio_process_completions_and_submit(s);
 275 }
 276
 277 static void ioq_init(LaioQueue *io_q)
 278 {
 279     QSIMPLEQ_INIT(&io_q->pending);
 280     io_q->plugged = 0;
 281     io_q->in_queue = 0;
 282     io_q->in_flight = 0;
 283     io_q->blocked = false;
 284 }
 285
 286 static void ioq_submit(LinuxAioState *s)
 287 {
 288     int ret, len;
 289     struct qemu_laiocb *aiocb;
 290     struct iocb *iocbs[MAX_EVENTS];
 291     QSIMPLEQ_HEAD(, qemu_laiocb) completed;
 292
 293     do {
 294         if (s->io_q.in_flight >= MAX_EVENTS) {
 295             break;
 296         }
 297         len = 0;
 298         QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
 299             iocbs[len++] = &aiocb->iocb;
 300             if (s->io_q.in_flight + len >= MAX_EVENTS) {
 301                 break;
 302             }
 303         }
 304
 305         ret = io_submit(s->ctx, len, iocbs);
 306         if (ret == -EAGAIN) {
 307             break;
 308         }
 309         if (ret < 0) {
 310             /* Fail the first request, retry the rest */
 311             aiocb = QSIMPLEQ_FIRST(&s->io_q.pending);
 312             QSIMPLEQ_REMOVE_HEAD(&s->io_q.pending, next);
 313             s->io_q.in_queue--;
 314             aiocb->ret = ret;
 315             qemu_laio_process_completion(aiocb);
 316             continue;
 317         }
 318
 319         s->io_q.in_flight += ret;
 320         s->io_q.in_queue  -= ret;
 321         aiocb = container_of(iocbs[ret - 1], struct qemu_laiocb, iocb);
 322         QSIMPLEQ_SPLIT_AFTER(&s->io_q.pending, aiocb, next, &completed);
 323     } while (ret == len && !QSIMPLEQ_EMPTY(&s->io_q.pending));
 324     s->io_q.blocked = (s->io_q.in_queue > 0);
 325
 326     if (s->io_q.in_flight) {
 327         /* We can try to complete something just right away if there are
 328          * still requests in-flight. */
 329         qemu_laio_process_completions(s);
 330         /*
 331          * Even we have completed everything (in_flight == 0), the queue can
 332          * have still pended requests (in_queue > 0).  We do not attempt to
 333          * repeat submission to avoid IO hang.  The reason is simple: s->e is
 334          * still set and completion callback will be called shortly and all
 335          * pended requests will be submitted from there.
 336          */
 337     }
 338 }
 339
 340 static uint64_t laio_max_batch(LinuxAioState *s, uint64_t dev_max_batch)
 341 {
 342     uint64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
 343
 344     /*
 345      * AIO context can be shared between multiple block devices, so
 346      * `dev_max_batch` allows reducing the batch size for latency-sensitive
 347      * devices.
 348      */
 349     max_batch = MIN_NON_ZERO(dev_max_batch, max_batch);
 350
 351     /* limit the batch with the number of available events */
 352     max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);
 353
 354     return max_batch;
 355 }
 356
 357 void laio_io_plug(BlockDriverState *bs, LinuxAioState *s)
 358 {
 359     s->io_q.plugged++;
 360 }
 361
 362 void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s,
 363                     uint64_t dev_max_batch)
 364 {
 365     assert(s->io_q.plugged);
 366     if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) ||
 367         (--s->io_q.plugged == 0 &&
 368          !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) {
 369         ioq_submit(s);
 370     }
 371 }
 372
 373 static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
 374                           int type, uint64_t dev_max_batch)
 375 {
 376     LinuxAioState *s = laiocb->ctx;
 377     struct iocb *iocbs = &laiocb->iocb;
 378     QEMUIOVector *qiov = laiocb->qiov;
 379
 380     switch (type) {
 381     case QEMU_AIO_WRITE:
 382         io_prep_pwritev(iocbs, fd, qiov->iov, qiov->niov, offset);
 383         break;
 384     case QEMU_AIO_READ:
 385         io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
 386         break;
 387     /* Currently Linux kernel does not support other operations */
 388     default:
 389         fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
 390                         __func__, type);
 391         return -EIO;
 392     }
 393     io_set_eventfd(&laiocb->iocb, event_notifier_get_fd(&s->e));
 394
 395     QSIMPLEQ_INSERT_TAIL(&s->io_q.pending, laiocb, next);
 396     s->io_q.in_queue++;
 397     if (!s->io_q.blocked &&
 398         (!s->io_q.plugged ||
 399          s->io_q.in_queue >= laio_max_batch(s, dev_max_batch))) {
 400         ioq_submit(s);
 401     }
 402
 403     return 0;
 404 }
 405
 406 int coroutine_fn laio_co_submit(BlockDriverState *bs, LinuxAioState *s, int fd,
 407                                 uint64_t offset, QEMUIOVector *qiov, int type,
 408                                 uint64_t dev_max_batch)
 409 {
 410     int ret;
 411     struct qemu_laiocb laiocb = {
 412         .co         = qemu_coroutine_self(),
 413         .nbytes     = qiov->size,
 414         .ctx        = s,
 415         .ret        = -EINPROGRESS,
 416         .is_read    = (type == QEMU_AIO_READ),
 417         .qiov       = qiov,
 418     };
 419
 420     ret = laio_do_submit(fd, &laiocb, offset, type, dev_max_batch);
 421     if (ret < 0) {
 422         return ret;
 423     }
 424
 425     if (laiocb.ret == -EINPROGRESS) {
 426         qemu_coroutine_yield();
 427     }
 428     return laiocb.ret;
 429 }
 430
 431 void laio_detach_aio_context(LinuxAioState *s, AioContext *old_context)
 432 {
 433     aio_set_event_notifier(old_context, &s->e, false, NULL, NULL, NULL);
 434     qemu_bh_delete(s->completion_bh);
 435     s->aio_context = NULL;
 436 }
 437
 438 void laio_attach_aio_context(LinuxAioState *s, AioContext *new_context)
 439 {
 440     s->aio_context = new_context;
 441     s->completion_bh = aio_bh_new(new_context, qemu_laio_completion_bh, s);
 442     aio_set_event_notifier(new_context, &s->e, false,
 443                            qemu_laio_completion_cb,
 444                            qemu_laio_poll_cb,
 445                            qemu_laio_poll_ready);
 446 }
 447
 448 LinuxAioState *laio_init(Error **errp)
 449 {
 450     int rc;
 451     LinuxAioState *s;
 452
 453     s = g_malloc0(sizeof(*s));
 454     rc = event_notifier_init(&s->e, false);
 455     if (rc < 0) {
 456         error_setg_errno(errp, -rc, "failed to to initialize event notifier");
 457         goto out_free_state;
 458     }
 459
 460     rc = io_setup(MAX_EVENTS, &s->ctx);
 461     if (rc < 0) {
 462         error_setg_errno(errp, -rc, "failed to create linux AIO context");
 463         goto out_close_efd;
 464     }
 465
 466     ioq_init(&s->io_q);
 467
 468     return s;
 469
 470 out_close_efd:
 471     event_notifier_cleanup(&s->e);
 472 out_free_state:
 473     g_free(s);
 474     return NULL;
 475 }
 476
 477 void laio_cleanup(LinuxAioState *s)
 478 {
 479     event_notifier_cleanup(&s->e);
 480
 481     if (io_destroy(s->ctx) != 0) {
 482         fprintf(stderr, "%s: destroy AIO context %p failed\n",
 483                         __func__, &s->ctx);
 484     }
 485     g_free(s);
 486 }