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 #include "sysemu/block-backend.h"
  19
  20 /* Only used for assertions.  */
  21 #include "qemu/coroutine_int.h"
  22
  23 #include <libaio.h>
  24
  25 /*
  26  * Queue size (per-device).
  27  *
  28  * XXX: eventually we need to communicate this to the guest and/or make it
  29  *      tunable by the guest.  If we get more outstanding requests at a time
  30  *      than this we will get EAGAIN from io_submit which is communicated to
  31  *      the guest as an I/O error.
  32  */
  33 #define MAX_EVENTS 1024
  34
  35 /* Maximum number of requests in a batch. (default value) */
  36 #define DEFAULT_MAX_BATCH 32
  37
  38 struct qemu_laiocb {
  39     Coroutine *co;
  40     LinuxAioState *ctx;
  41     struct iocb iocb;
  42     ssize_t ret;
  43     size_t nbytes;
  44     QEMUIOVector *qiov;
  45     bool is_read;
  46     QSIMPLEQ_ENTRY(qemu_laiocb) next;
  47 };
  48
  49 typedef struct {
  50     unsigned int in_queue;
  51     unsigned int in_flight;
  52     bool blocked;
  53     QSIMPLEQ_HEAD(, qemu_laiocb) pending;
  54 } LaioQueue;
  55
  56 struct LinuxAioState {
  57     AioContext *aio_context;
  58
  59     io_context_t ctx;
  60     EventNotifier e;
  61
  62     /* No locking required, only accessed from AioContext home thread */
  63     LaioQueue io_q;
  64     QEMUBH *completion_bh;
  65     int event_idx;
  66     int event_max;
  67 };
  68
  69 static void ioq_submit(LinuxAioState *s);
  70
  71 static inline ssize_t io_event_ret(struct io_event *ev)
  72 {
  73     return (ssize_t)(((uint64_t)ev->res2 << 32) | ev->res);
  74 }
  75
  76 /*
  77  * Completes an AIO request.
  78  */
  79 static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
  80 {
  81     int ret;
  82
  83     ret = laiocb->ret;
  84     if (ret != -ECANCELED) {
  85         if (ret == laiocb->nbytes) {
  86             ret = 0;
  87         } else if (ret >= 0) {
  88             /* Short reads mean EOF, pad with zeros. */
  89             if (laiocb->is_read) {
  90                 qemu_iovec_memset(laiocb->qiov, ret, 0,
  91                     laiocb->qiov->size - ret);
  92             } else {
  93                 ret = -ENOSPC;
  94             }
  95         }
  96     }
  97
  98     laiocb->ret = ret;
  99
 100     /*
 101      * If the coroutine is already entered it must be in ioq_submit() and
 102      * will notice laio->ret has been filled in when it eventually runs
 103      * later.  Coroutines cannot be entered recursively so avoid doing
 104      * that!
 105      */
 106     assert(laiocb->co->ctx == laiocb->ctx->aio_context);
 107     if (!qemu_coroutine_entered(laiocb->co)) {
 108         aio_co_wake(laiocb->co);
 109     }
 110 }
 111
 112 /**
 113  * aio_ring buffer which is shared between userspace and kernel.
 114  *
 115  * This copied from linux/fs/aio.c, common header does not exist
 116  * but AIO exists for ages so we assume ABI is stable.
 117  */
 118 struct aio_ring {
 119     unsigned    id;    /* kernel internal index number */
 120     unsigned    nr;    /* number of io_events */
 121     unsigned    head;  /* Written to by userland or by kernel. */
 122     unsigned    tail;
 123
 124     unsigned    magic;
 125     unsigned    compat_features;
 126     unsigned    incompat_features;
 127     unsigned    header_length;  /* size of aio_ring */
 128
 129     struct io_event io_events[];
 130 };
 131
 132 /**
 133  * io_getevents_peek:
 134  * @ctx: AIO context
 135  * @events: pointer on events array, output value
 136
 137  * Returns the number of completed events and sets a pointer
 138  * on events array.  This function does not update the internal
 139  * ring buffer, only reads head and tail.  When @events has been
 140  * processed io_getevents_commit() must be called.
 141  */
 142 static inline unsigned int io_getevents_peek(io_context_t ctx,
 143                                              struct io_event **events)
 144 {
 145     struct aio_ring *ring = (struct aio_ring *)ctx;
 146     unsigned int head = ring->head, tail = ring->tail;
 147     unsigned int nr;
 148
 149     nr = tail >= head ? tail - head : ring->nr - head;
 150     *events = ring->io_events + head;
 151     /* To avoid speculative loads of s->events[i] before observing tail.
 152        Paired with smp_wmb() inside linux/fs/aio.c: aio_complete(). */
 153     smp_rmb();
 154
 155     return nr;
 156 }
 157
 158 /**
 159  * io_getevents_commit:
 160  * @ctx: AIO context
 161  * @nr: the number of events on which head should be advanced
 162  *
 163  * Advances head of a ring buffer.
 164  */
 165 static inline void io_getevents_commit(io_context_t ctx, unsigned int nr)
 166 {
 167     struct aio_ring *ring = (struct aio_ring *)ctx;
 168
 169     if (nr) {
 170         ring->head = (ring->head + nr) % ring->nr;
 171     }
 172 }
 173
 174 /**
 175  * io_getevents_advance_and_peek:
 176  * @ctx: AIO context
 177  * @events: pointer on events array, output value
 178  * @nr: the number of events on which head should be advanced
 179  *
 180  * Advances head of a ring buffer and returns number of elements left.
 181  */
 182 static inline unsigned int
 183 io_getevents_advance_and_peek(io_context_t ctx,
 184                               struct io_event **events,
 185                               unsigned int nr)
 186 {
 187     io_getevents_commit(ctx, nr);
 188     return io_getevents_peek(ctx, events);
 189 }
 190
 191 /**
 192  * qemu_laio_process_completions:
 193  * @s: AIO state
 194  *
 195  * Fetches completed I/O requests and invokes their callbacks.
 196  *
 197  * The function is somewhat tricky because it supports nested event loops, for
 198  * example when a request callback invokes aio_poll().  In order to do this,
 199  * indices are kept in LinuxAioState.  Function schedules BH completion so it
 200  * can be called again in a nested event loop.  When there are no events left
 201  * to complete the BH is being canceled.
 202  */
 203 static void qemu_laio_process_completions(LinuxAioState *s)
 204 {
 205     struct io_event *events;
 206
 207     /* Reschedule so nested event loops see currently pending completions */
 208     qemu_bh_schedule(s->completion_bh);
 209
 210     while ((s->event_max = io_getevents_advance_and_peek(s->ctx, &events,
 211                                                          s->event_idx))) {
 212         for (s->event_idx = 0; s->event_idx < s->event_max; ) {
 213             struct iocb *iocb = events[s->event_idx].obj;
 214             struct qemu_laiocb *laiocb =
 215                 container_of(iocb, struct qemu_laiocb, iocb);
 216
 217             laiocb->ret = io_event_ret(&events[s->event_idx]);
 218
 219             /* Change counters one-by-one because we can be nested. */
 220             s->io_q.in_flight--;
 221             s->event_idx++;
 222             qemu_laio_process_completion(laiocb);
 223         }
 224     }
 225
 226     qemu_bh_cancel(s->completion_bh);
 227
 228     /* If we are nested we have to notify the level above that we are done
 229      * by setting event_max to zero, upper level will then jump out of it's
 230      * own `for` loop.  If we are the last all counters droped to zero. */
 231     s->event_max = 0;
 232     s->event_idx = 0;
 233 }
 234
 235 static void qemu_laio_process_completions_and_submit(LinuxAioState *s)
 236 {
 237     qemu_laio_process_completions(s);
 238
 239     if (!QSIMPLEQ_EMPTY(&s->io_q.pending)) {
 240         ioq_submit(s);
 241     }
 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->in_queue = 0;
 281     io_q->in_flight = 0;
 282     io_q->blocked = false;
 283 }
 284
 285 static void ioq_submit(LinuxAioState *s)
 286 {
 287     int ret, len;
 288     struct qemu_laiocb *aiocb;
 289     struct iocb *iocbs[MAX_EVENTS];
 290     QSIMPLEQ_HEAD(, qemu_laiocb) completed;
 291
 292     do {
 293         if (s->io_q.in_flight >= MAX_EVENTS) {
 294             break;
 295         }
 296         len = 0;
 297         QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
 298             iocbs[len++] = &aiocb->iocb;
 299             if (s->io_q.in_flight + len >= MAX_EVENTS) {
 300                 break;
 301             }
 302         }
 303
 304         ret = io_submit(s->ctx, len, iocbs);
 305         if (ret == -EAGAIN) {
 306             break;
 307         }
 308         if (ret < 0) {
 309             /* Fail the first request, retry the rest */
 310             aiocb = QSIMPLEQ_FIRST(&s->io_q.pending);
 311             QSIMPLEQ_REMOVE_HEAD(&s->io_q.pending, next);
 312             s->io_q.in_queue--;
 313             aiocb->ret = ret;
 314             qemu_laio_process_completion(aiocb);
 315             continue;
 316         }
 317
 318         s->io_q.in_flight += ret;
 319         s->io_q.in_queue  -= ret;
 320         aiocb = container_of(iocbs[ret - 1], struct qemu_laiocb, iocb);
 321         QSIMPLEQ_SPLIT_AFTER(&s->io_q.pending, aiocb, next, &completed);
 322     } while (ret == len && !QSIMPLEQ_EMPTY(&s->io_q.pending));
 323     s->io_q.blocked = (s->io_q.in_queue > 0);
 324
 325     if (s->io_q.in_flight) {
 326         /* We can try to complete something just right away if there are
 327          * still requests in-flight. */
 328         qemu_laio_process_completions(s);
 329         /*
 330          * Even we have completed everything (in_flight == 0), the queue can
 331          * have still pended requests (in_queue > 0).  We do not attempt to
 332          * repeat submission to avoid IO hang.  The reason is simple: s->e is
 333          * still set and completion callback will be called shortly and all
 334          * pended requests will be submitted from there.
 335          */
 336     }
 337 }
 338
 339 static uint64_t laio_max_batch(LinuxAioState *s, uint64_t dev_max_batch)
 340 {
 341     uint64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
 342
 343     /*
 344      * AIO context can be shared between multiple block devices, so
 345      * `dev_max_batch` allows reducing the batch size for latency-sensitive
 346      * devices.
 347      */
 348     max_batch = MIN_NON_ZERO(dev_max_batch, max_batch);
 349
 350     /* limit the batch with the number of available events */
 351     max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);
 352
 353     return max_batch;
 354 }
 355
 356 static void laio_unplug_fn(void *opaque)
 357 {
 358     LinuxAioState *s = opaque;
 359
 360     if (!s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
 361         ioq_submit(s);
 362     }
 363 }
 364
 365 static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
 366                           int type, uint64_t dev_max_batch)
 367 {
 368     LinuxAioState *s = laiocb->ctx;
 369     struct iocb *iocbs = &laiocb->iocb;
 370     QEMUIOVector *qiov = laiocb->qiov;
 371
 372     switch (type) {
 373     case QEMU_AIO_WRITE:
 374         io_prep_pwritev(iocbs, fd, qiov->iov, qiov->niov, offset);
 375         break;
 376     case QEMU_AIO_ZONE_APPEND:
 377         io_prep_pwritev(iocbs, fd, qiov->iov, qiov->niov, offset);
 378         break;
 379     case QEMU_AIO_READ:
 380         io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
 381         break;
 382     /* Currently Linux kernel does not support other operations */
 383     default:
 384         fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
 385                         __func__, type);
 386         return -EIO;
 387     }
 388     io_set_eventfd(&laiocb->iocb, event_notifier_get_fd(&s->e));
 389
 390     QSIMPLEQ_INSERT_TAIL(&s->io_q.pending, laiocb, next);
 391     s->io_q.in_queue++;
 392     if (!s->io_q.blocked) {
 393         if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch)) {
 394             ioq_submit(s);
 395         } else {
 396             blk_io_plug_call(laio_unplug_fn, s);
 397         }
 398     }
 399
 400     return 0;
 401 }
 402
 403 int coroutine_fn laio_co_submit(int fd, uint64_t offset, QEMUIOVector *qiov,
 404                                 int type, uint64_t dev_max_batch)
 405 {
 406     int ret;
 407     AioContext *ctx = qemu_get_current_aio_context();
 408     struct qemu_laiocb laiocb = {
 409         .co         = qemu_coroutine_self(),
 410         .nbytes     = qiov->size,
 411         .ctx        = aio_get_linux_aio(ctx),
 412         .ret        = -EINPROGRESS,
 413         .is_read    = (type == QEMU_AIO_READ),
 414         .qiov       = qiov,
 415     };
 416
 417     ret = laio_do_submit(fd, &laiocb, offset, type, dev_max_batch);
 418     if (ret < 0) {
 419         return ret;
 420     }
 421
 422     if (laiocb.ret == -EINPROGRESS) {
 423         qemu_coroutine_yield();
 424     }
 425     return laiocb.ret;
 426 }
 427
 428 void laio_detach_aio_context(LinuxAioState *s, AioContext *old_context)
 429 {
 430     aio_set_event_notifier(old_context, &s->e, NULL, NULL, NULL);
 431     qemu_bh_delete(s->completion_bh);
 432     s->aio_context = NULL;
 433 }
 434
 435 void laio_attach_aio_context(LinuxAioState *s, AioContext *new_context)
 436 {
 437     s->aio_context = new_context;
 438     s->completion_bh = aio_bh_new(new_context, qemu_laio_completion_bh, s);
 439     aio_set_event_notifier(new_context, &s->e,
 440                            qemu_laio_completion_cb,
 441                            qemu_laio_poll_cb,
 442                            qemu_laio_poll_ready);
 443 }
 444
 445 LinuxAioState *laio_init(Error **errp)
 446 {
 447     int rc;
 448     LinuxAioState *s;
 449
 450     s = g_malloc0(sizeof(*s));
 451     rc = event_notifier_init(&s->e, false);
 452     if (rc < 0) {
 453         error_setg_errno(errp, -rc, "failed to initialize event notifier");
 454         goto out_free_state;
 455     }
 456
 457     rc = io_setup(MAX_EVENTS, &s->ctx);
 458     if (rc < 0) {
 459         error_setg_errno(errp, -rc, "failed to create linux AIO context");
 460         goto out_close_efd;
 461     }
 462
 463     ioq_init(&s->io_q);
 464
 465     return s;
 466
 467 out_close_efd:
 468     event_notifier_cleanup(&s->e);
 469 out_free_state:
 470     g_free(s);
 471     return NULL;
 472 }
 473
 474 void laio_cleanup(LinuxAioState *s)
 475 {
 476     event_notifier_cleanup(&s->e);
 477
 478     if (io_destroy(s->ctx) != 0) {
 479         fprintf(stderr, "%s: destroy AIO context %p failed\n",
 480                         __func__, &s->ctx);
 481     }
 482     g_free(s);
 483 }