util/async.c

   1 /*
   2  * Data plane event loop
   3  *
   4  * Copyright (c) 2003-2008 Fabrice Bellard
   5  * Copyright (c) 2009-2017 QEMU contributors
   6  *
   7  * Permission is hereby granted, free of charge, to any person obtaining a copy
   8  * of this software and associated documentation files (the "Software"), to deal
   9  * in the Software without restriction, including without limitation the rights
  10  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  11  * copies of the Software, and to permit persons to whom the Software is
  12  * furnished to do so, subject to the following conditions:
  13  *
  14  * The above copyright notice and this permission notice shall be included in
  15  * all copies or substantial portions of the Software.
  16  *
  17  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  18  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  19  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  20  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  21  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  22  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  23  * THE SOFTWARE.
  24  */
  25
  26 #include "qemu/osdep.h"
  27 #include "qapi/error.h"
  28 #include "block/aio.h"
  29 #include "block/thread-pool.h"
  30 #include "qemu/main-loop.h"
  31 #include "qemu/atomic.h"
  32 #include "qemu/rcu_queue.h"
  33 #include "block/raw-aio.h"
  34 #include "qemu/coroutine_int.h"
  35 #include "trace.h"
  36
  37 /***********************************************************/
  38 /* bottom halves (can be seen as timers which expire ASAP) */
  39
  40 /* QEMUBH::flags values */
  41 enum {
  42     /* Already enqueued and waiting for aio_bh_poll() */
  43     BH_PENDING   = (1 << 0),
  44
  45     /* Invoke the callback */
  46     BH_SCHEDULED = (1 << 1),
  47
  48     /* Delete without invoking callback */
  49     BH_DELETED   = (1 << 2),
  50
  51     /* Delete after invoking callback */
  52     BH_ONESHOT   = (1 << 3),
  53
  54     /* Schedule periodically when the event loop is idle */
  55     BH_IDLE      = (1 << 4),
  56 };
  57
  58 struct QEMUBH {
  59     AioContext *ctx;
  60     const char *name;
  61     QEMUBHFunc *cb;
  62     void *opaque;
  63     QSLIST_ENTRY(QEMUBH) next;
  64     unsigned flags;
  65 };
  66
  67 /* Called concurrently from any thread */
  68 static void aio_bh_enqueue(QEMUBH *bh, unsigned new_flags)
  69 {
  70     AioContext *ctx = bh->ctx;
  71     unsigned old_flags;
  72
  73     /*
  74      * The memory barrier implicit in qatomic_fetch_or makes sure that:
  75      * 1. idle & any writes needed by the callback are done before the
  76      *    locations are read in the aio_bh_poll.
  77      * 2. ctx is loaded before the callback has a chance to execute and bh
  78      *    could be freed.
  79      */
  80     old_flags = qatomic_fetch_or(&bh->flags, BH_PENDING | new_flags);
  81     if (!(old_flags & BH_PENDING)) {
  82         QSLIST_INSERT_HEAD_ATOMIC(&ctx->bh_list, bh, next);
  83     }
  84
  85     aio_notify(ctx);
  86 }
  87
  88 /* Only called from aio_bh_poll() and aio_ctx_finalize() */
  89 static QEMUBH *aio_bh_dequeue(BHList *head, unsigned *flags)
  90 {
  91     QEMUBH *bh = QSLIST_FIRST_RCU(head);
  92
  93     if (!bh) {
  94         return NULL;
  95     }
  96
  97     QSLIST_REMOVE_HEAD(head, next);
  98
  99     /*
 100      * The qatomic_and is paired with aio_bh_enqueue().  The implicit memory
 101      * barrier ensures that the callback sees all writes done by the scheduling
 102      * thread.  It also ensures that the scheduling thread sees the cleared
 103      * flag before bh->cb has run, and thus will call aio_notify again if
 104      * necessary.
 105      */
 106     *flags = qatomic_fetch_and(&bh->flags,
 107                               ~(BH_PENDING | BH_SCHEDULED | BH_IDLE));
 108     return bh;
 109 }
 110
 111 void aio_bh_schedule_oneshot_full(AioContext *ctx, QEMUBHFunc *cb,
 112                                   void *opaque, const char *name)
 113 {
 114     QEMUBH *bh;
 115     bh = g_new(QEMUBH, 1);
 116     *bh = (QEMUBH){
 117         .ctx = ctx,
 118         .cb = cb,
 119         .opaque = opaque,
 120         .name = name,
 121     };
 122     aio_bh_enqueue(bh, BH_SCHEDULED | BH_ONESHOT);
 123 }
 124
 125 QEMUBH *aio_bh_new_full(AioContext *ctx, QEMUBHFunc *cb, void *opaque,
 126                         const char *name)
 127 {
 128     QEMUBH *bh;
 129     bh = g_new(QEMUBH, 1);
 130     *bh = (QEMUBH){
 131         .ctx = ctx,
 132         .cb = cb,
 133         .opaque = opaque,
 134         .name = name,
 135     };
 136     return bh;
 137 }
 138
 139 void aio_bh_call(QEMUBH *bh)
 140 {
 141     bh->cb(bh->opaque);
 142 }
 143
 144 /* Multiple occurrences of aio_bh_poll cannot be called concurrently. */
 145 int aio_bh_poll(AioContext *ctx)
 146 {
 147     BHListSlice slice;
 148     BHListSlice *s;
 149     int ret = 0;
 150
 151     QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
 152     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
 153
 154     while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
 155         QEMUBH *bh;
 156         unsigned flags;
 157
 158         bh = aio_bh_dequeue(&s->bh_list, &flags);
 159         if (!bh) {
 160             QSIMPLEQ_REMOVE_HEAD(&ctx->bh_slice_list, next);
 161             continue;
 162         }
 163
 164         if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
 165             /* Idle BHs don't count as progress */
 166             if (!(flags & BH_IDLE)) {
 167                 ret = 1;
 168             }
 169             aio_bh_call(bh);
 170         }
 171         if (flags & (BH_DELETED | BH_ONESHOT)) {
 172             g_free(bh);
 173         }
 174     }
 175
 176     return ret;
 177 }
 178
 179 void qemu_bh_schedule_idle(QEMUBH *bh)
 180 {
 181     aio_bh_enqueue(bh, BH_SCHEDULED | BH_IDLE);
 182 }
 183
 184 void qemu_bh_schedule(QEMUBH *bh)
 185 {
 186     aio_bh_enqueue(bh, BH_SCHEDULED);
 187 }
 188
 189 /* This func is async.
 190  */
 191 void qemu_bh_cancel(QEMUBH *bh)
 192 {
 193     qatomic_and(&bh->flags, ~BH_SCHEDULED);
 194 }
 195
 196 /* This func is async.The bottom half will do the delete action at the finial
 197  * end.
 198  */
 199 void qemu_bh_delete(QEMUBH *bh)
 200 {
 201     aio_bh_enqueue(bh, BH_DELETED);
 202 }
 203
 204 static int64_t aio_compute_bh_timeout(BHList *head, int timeout)
 205 {
 206     QEMUBH *bh;
 207
 208     QSLIST_FOREACH_RCU(bh, head, next) {
 209         if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
 210             if (bh->flags & BH_IDLE) {
 211                 /* idle bottom halves will be polled at least
 212                  * every 10ms */
 213                 timeout = 10000000;
 214             } else {
 215                 /* non-idle bottom halves will be executed
 216                  * immediately */
 217                 return 0;
 218             }
 219         }
 220     }
 221
 222     return timeout;
 223 }
 224
 225 int64_t
 226 aio_compute_timeout(AioContext *ctx)
 227 {
 228     BHListSlice *s;
 229     int64_t deadline;
 230     int timeout = -1;
 231
 232     timeout = aio_compute_bh_timeout(&ctx->bh_list, timeout);
 233     if (timeout == 0) {
 234         return 0;
 235     }
 236
 237     QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
 238         timeout = aio_compute_bh_timeout(&s->bh_list, timeout);
 239         if (timeout == 0) {
 240             return 0;
 241         }
 242     }
 243
 244     deadline = timerlistgroup_deadline_ns(&ctx->tlg);
 245     if (deadline == 0) {
 246         return 0;
 247     } else {
 248         return qemu_soonest_timeout(timeout, deadline);
 249     }
 250 }
 251
 252 static gboolean
 253 aio_ctx_prepare(GSource *source, gint    *timeout)
 254 {
 255     AioContext *ctx = (AioContext *) source;
 256
 257     qatomic_set(&ctx->notify_me, qatomic_read(&ctx->notify_me) | 1);
 258
 259     /*
 260      * Write ctx->notify_me before computing the timeout
 261      * (reading bottom half flags, etc.).  Pairs with
 262      * smp_mb in aio_notify().
 263      */
 264     smp_mb();
 265
 266     /* We assume there is no timeout already supplied */
 267     *timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
 268
 269     if (aio_prepare(ctx)) {
 270         *timeout = 0;
 271     }
 272
 273     return *timeout == 0;
 274 }
 275
 276 static gboolean
 277 aio_ctx_check(GSource *source)
 278 {
 279     AioContext *ctx = (AioContext *) source;
 280     QEMUBH *bh;
 281     BHListSlice *s;
 282
 283     /* Finish computing the timeout before clearing the flag.  */
 284     qatomic_store_release(&ctx->notify_me, qatomic_read(&ctx->notify_me) & ~1);
 285     aio_notify_accept(ctx);
 286
 287     QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {
 288         if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
 289             return true;
 290         }
 291     }
 292
 293     QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
 294         QSLIST_FOREACH_RCU(bh, &s->bh_list, next) {
 295             if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
 296                 return true;
 297             }
 298         }
 299     }
 300     return aio_pending(ctx) || (timerlistgroup_deadline_ns(&ctx->tlg) == 0);
 301 }
 302
 303 static gboolean
 304 aio_ctx_dispatch(GSource     *source,
 305                  GSourceFunc  callback,
 306                  gpointer     user_data)
 307 {
 308     AioContext *ctx = (AioContext *) source;
 309
 310     assert(callback == NULL);
 311     aio_dispatch(ctx);
 312     return true;
 313 }
 314
 315 static void
 316 aio_ctx_finalize(GSource     *source)
 317 {
 318     AioContext *ctx = (AioContext *) source;
 319     QEMUBH *bh;
 320     unsigned flags;
 321
 322     thread_pool_free(ctx->thread_pool);
 323
 324 #ifdef CONFIG_LINUX_AIO
 325     if (ctx->linux_aio) {
 326         laio_detach_aio_context(ctx->linux_aio, ctx);
 327         laio_cleanup(ctx->linux_aio);
 328         ctx->linux_aio = NULL;
 329     }
 330 #endif
 331
 332 #ifdef CONFIG_LINUX_IO_URING
 333     if (ctx->linux_io_uring) {
 334         luring_detach_aio_context(ctx->linux_io_uring, ctx);
 335         luring_cleanup(ctx->linux_io_uring);
 336         ctx->linux_io_uring = NULL;
 337     }
 338 #endif
 339
 340     assert(QSLIST_EMPTY(&ctx->scheduled_coroutines));
 341     qemu_bh_delete(ctx->co_schedule_bh);
 342
 343     /* There must be no aio_bh_poll() calls going on */
 344     assert(QSIMPLEQ_EMPTY(&ctx->bh_slice_list));
 345
 346     while ((bh = aio_bh_dequeue(&ctx->bh_list, &flags))) {
 347         /*
 348          * qemu_bh_delete() must have been called on BHs in this AioContext. In
 349          * many cases memory leaks, hangs, or inconsistent state occur when a
 350          * BH is leaked because something still expects it to run.
 351          *
 352          * If you hit this, fix the lifecycle of the BH so that
 353          * qemu_bh_delete() and any associated cleanup is called before the
 354          * AioContext is finalized.
 355          */
 356         if (unlikely(!(flags & BH_DELETED))) {
 357             fprintf(stderr, "%s: BH '%s' leaked, aborting...\n",
 358                     __func__, bh->name);
 359             abort();
 360         }
 361
 362         g_free(bh);
 363     }
 364
 365     aio_set_event_notifier(ctx, &ctx->notifier, false, NULL, NULL);
 366     event_notifier_cleanup(&ctx->notifier);
 367     qemu_rec_mutex_destroy(&ctx->lock);
 368     qemu_lockcnt_destroy(&ctx->list_lock);
 369     timerlistgroup_deinit(&ctx->tlg);
 370     aio_context_destroy(ctx);
 371 }
 372
 373 static GSourceFuncs aio_source_funcs = {
 374     aio_ctx_prepare,
 375     aio_ctx_check,
 376     aio_ctx_dispatch,
 377     aio_ctx_finalize
 378 };
 379
 380 GSource *aio_get_g_source(AioContext *ctx)
 381 {
 382     aio_context_use_g_source(ctx);
 383     g_source_ref(&ctx->source);
 384     return &ctx->source;
 385 }
 386
 387 ThreadPool *aio_get_thread_pool(AioContext *ctx)
 388 {
 389     if (!ctx->thread_pool) {
 390         ctx->thread_pool = thread_pool_new(ctx);
 391     }
 392     return ctx->thread_pool;
 393 }
 394
 395 #ifdef CONFIG_LINUX_AIO
 396 LinuxAioState *aio_setup_linux_aio(AioContext *ctx, Error **errp)
 397 {
 398     if (!ctx->linux_aio) {
 399         ctx->linux_aio = laio_init(errp);
 400         if (ctx->linux_aio) {
 401             laio_attach_aio_context(ctx->linux_aio, ctx);
 402         }
 403     }
 404     return ctx->linux_aio;
 405 }
 406
 407 LinuxAioState *aio_get_linux_aio(AioContext *ctx)
 408 {
 409     assert(ctx->linux_aio);
 410     return ctx->linux_aio;
 411 }
 412 #endif
 413
 414 #ifdef CONFIG_LINUX_IO_URING
 415 LuringState *aio_setup_linux_io_uring(AioContext *ctx, Error **errp)
 416 {
 417     if (ctx->linux_io_uring) {
 418         return ctx->linux_io_uring;
 419     }
 420
 421     ctx->linux_io_uring = luring_init(errp);
 422     if (!ctx->linux_io_uring) {
 423         return NULL;
 424     }
 425
 426     luring_attach_aio_context(ctx->linux_io_uring, ctx);
 427     return ctx->linux_io_uring;
 428 }
 429
 430 LuringState *aio_get_linux_io_uring(AioContext *ctx)
 431 {
 432     assert(ctx->linux_io_uring);
 433     return ctx->linux_io_uring;
 434 }
 435 #endif
 436
 437 void aio_notify(AioContext *ctx)
 438 {
 439     /*
 440      * Write e.g. bh->flags before writing ctx->notified.  Pairs with smp_mb in
 441      * aio_notify_accept.
 442      */
 443     smp_wmb();
 444     qatomic_set(&ctx->notified, true);
 445
 446     /*
 447      * Write ctx->notified before reading ctx->notify_me.  Pairs
 448      * with smp_mb in aio_ctx_prepare or aio_poll.
 449      */
 450     smp_mb();
 451     if (qatomic_read(&ctx->notify_me)) {
 452         event_notifier_set(&ctx->notifier);
 453     }
 454 }
 455
 456 void aio_notify_accept(AioContext *ctx)
 457 {
 458     qatomic_set(&ctx->notified, false);
 459
 460     /*
 461      * Write ctx->notified before reading e.g. bh->flags.  Pairs with smp_wmb
 462      * in aio_notify.
 463      */
 464     smp_mb();
 465 }
 466
 467 static void aio_timerlist_notify(void *opaque, QEMUClockType type)
 468 {
 469     aio_notify(opaque);
 470 }
 471
 472 static void aio_context_notifier_cb(EventNotifier *e)
 473 {
 474     AioContext *ctx = container_of(e, AioContext, notifier);
 475
 476     event_notifier_test_and_clear(&ctx->notifier);
 477 }
 478
 479 /* Returns true if aio_notify() was called (e.g. a BH was scheduled) */
 480 static bool aio_context_notifier_poll(void *opaque)
 481 {
 482     EventNotifier *e = opaque;
 483     AioContext *ctx = container_of(e, AioContext, notifier);
 484
 485     return qatomic_read(&ctx->notified);
 486 }
 487
 488 static void co_schedule_bh_cb(void *opaque)
 489 {
 490     AioContext *ctx = opaque;
 491     QSLIST_HEAD(, Coroutine) straight, reversed;
 492
 493     QSLIST_MOVE_ATOMIC(&reversed, &ctx->scheduled_coroutines);
 494     QSLIST_INIT(&straight);
 495
 496     while (!QSLIST_EMPTY(&reversed)) {
 497         Coroutine *co = QSLIST_FIRST(&reversed);
 498         QSLIST_REMOVE_HEAD(&reversed, co_scheduled_next);
 499         QSLIST_INSERT_HEAD(&straight, co, co_scheduled_next);
 500     }
 501
 502     while (!QSLIST_EMPTY(&straight)) {
 503         Coroutine *co = QSLIST_FIRST(&straight);
 504         QSLIST_REMOVE_HEAD(&straight, co_scheduled_next);
 505         trace_aio_co_schedule_bh_cb(ctx, co);
 506         aio_context_acquire(ctx);
 507
 508         /* Protected by write barrier in qemu_aio_coroutine_enter */
 509         qatomic_set(&co->scheduled, NULL);
 510         qemu_aio_coroutine_enter(ctx, co);
 511         aio_context_release(ctx);
 512     }
 513 }
 514
 515 AioContext *aio_context_new(Error **errp)
 516 {
 517     int ret;
 518     AioContext *ctx;
 519
 520     ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
 521     QSLIST_INIT(&ctx->bh_list);
 522     QSIMPLEQ_INIT(&ctx->bh_slice_list);
 523     aio_context_setup(ctx);
 524
 525     ret = event_notifier_init(&ctx->notifier, false);
 526     if (ret < 0) {
 527         error_setg_errno(errp, -ret, "Failed to initialize event notifier");
 528         goto fail;
 529     }
 530     g_source_set_can_recurse(&ctx->source, true);
 531     qemu_lockcnt_init(&ctx->list_lock);
 532
 533     ctx->co_schedule_bh = aio_bh_new(ctx, co_schedule_bh_cb, ctx);
 534     QSLIST_INIT(&ctx->scheduled_coroutines);
 535
 536     aio_set_event_notifier(ctx, &ctx->notifier,
 537                            false,
 538                            aio_context_notifier_cb,
 539                            aio_context_notifier_poll);
 540 #ifdef CONFIG_LINUX_AIO
 541     ctx->linux_aio = NULL;
 542 #endif
 543
 544 #ifdef CONFIG_LINUX_IO_URING
 545     ctx->linux_io_uring = NULL;
 546 #endif
 547
 548     ctx->thread_pool = NULL;
 549     qemu_rec_mutex_init(&ctx->lock);
 550     timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
 551
 552     ctx->poll_ns = 0;
 553     ctx->poll_max_ns = 0;
 554     ctx->poll_grow = 0;
 555     ctx->poll_shrink = 0;
 556
 557     ctx->aio_max_batch = 0;
 558
 559     return ctx;
 560 fail:
 561     g_source_destroy(&ctx->source);
 562     return NULL;
 563 }
 564
 565 void aio_co_schedule(AioContext *ctx, Coroutine *co)
 566 {
 567     trace_aio_co_schedule(ctx, co);
 568     const char *scheduled = qatomic_cmpxchg(&co->scheduled, NULL,
 569                                            __func__);
 570
 571     if (scheduled) {
 572         fprintf(stderr,
 573                 "%s: Co-routine was already scheduled in '%s'\n",
 574                 __func__, scheduled);
 575         abort();
 576     }
 577
 578     /* The coroutine might run and release the last ctx reference before we
 579      * invoke qemu_bh_schedule().  Take a reference to keep ctx alive until
 580      * we're done.
 581      */
 582     aio_context_ref(ctx);
 583
 584     QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines,
 585                               co, co_scheduled_next);
 586     qemu_bh_schedule(ctx->co_schedule_bh);
 587
 588     aio_context_unref(ctx);
 589 }
 590
 591 typedef struct AioCoRescheduleSelf {
 592     Coroutine *co;
 593     AioContext *new_ctx;
 594 } AioCoRescheduleSelf;
 595
 596 static void aio_co_reschedule_self_bh(void *opaque)
 597 {
 598     AioCoRescheduleSelf *data = opaque;
 599     aio_co_schedule(data->new_ctx, data->co);
 600 }
 601
 602 void coroutine_fn aio_co_reschedule_self(AioContext *new_ctx)
 603 {
 604     AioContext *old_ctx = qemu_get_current_aio_context();
 605
 606     if (old_ctx != new_ctx) {
 607         AioCoRescheduleSelf data = {
 608             .co = qemu_coroutine_self(),
 609             .new_ctx = new_ctx,
 610         };
 611         /*
 612          * We can't directly schedule the coroutine in the target context
 613          * because this would be racy: The other thread could try to enter the
 614          * coroutine before it has yielded in this one.
 615          */
 616         aio_bh_schedule_oneshot(old_ctx, aio_co_reschedule_self_bh, &data);
 617         qemu_coroutine_yield();
 618     }
 619 }
 620
 621 void aio_co_wake(struct Coroutine *co)
 622 {
 623     AioContext *ctx;
 624
 625     /* Read coroutine before co->ctx.  Matches smp_wmb in
 626      * qemu_coroutine_enter.
 627      */
 628     smp_read_barrier_depends();
 629     ctx = qatomic_read(&co->ctx);
 630
 631     aio_co_enter(ctx, co);
 632 }
 633
 634 void aio_co_enter(AioContext *ctx, struct Coroutine *co)
 635 {
 636     if (ctx != qemu_get_current_aio_context()) {
 637         aio_co_schedule(ctx, co);
 638         return;
 639     }
 640
 641     if (qemu_in_coroutine()) {
 642         Coroutine *self = qemu_coroutine_self();
 643         assert(self != co);
 644         QSIMPLEQ_INSERT_TAIL(&self->co_queue_wakeup, co, co_queue_next);
 645     } else {
 646         aio_context_acquire(ctx);
 647         qemu_aio_coroutine_enter(ctx, co);
 648         aio_context_release(ctx);
 649     }
 650 }
 651
 652 void aio_context_ref(AioContext *ctx)
 653 {
 654     g_source_ref(&ctx->source);
 655 }
 656
 657 void aio_context_unref(AioContext *ctx)
 658 {
 659     g_source_unref(&ctx->source);
 660 }
 661
 662 void aio_context_acquire(AioContext *ctx)
 663 {
 664     qemu_rec_mutex_lock(&ctx->lock);
 665 }
 666
 667 void aio_context_release(AioContext *ctx)
 668 {
 669     qemu_rec_mutex_unlock(&ctx->lock);
 670 }
 671
 672 static __thread AioContext *my_aiocontext;
 673
 674 AioContext *qemu_get_current_aio_context(void)
 675 {
 676     if (my_aiocontext) {
 677         return my_aiocontext;
 678     }
 679     if (qemu_mutex_iothread_locked()) {
 680         /* Possibly in a vCPU thread.  */
 681         return qemu_get_aio_context();
 682     }
 683     return NULL;
 684 }
 685
 686 void qemu_set_current_aio_context(AioContext *ctx)
 687 {
 688     assert(!my_aiocontext);
 689     my_aiocontext = ctx;
 690 }