qemu

aio-posix.c (23388B)

---

 /*
  * QEMU aio implementation
  *
  * Copyright IBM, Corp. 2008
  *
  * Authors:
  *  Anthony Liguori   <aliguori@us.ibm.com>
  *
  * This work is licensed under the terms of the GNU GPL, version 2.  See
  * the COPYING file in the top-level directory.
  *
  * Contributions after 2012-01-13 are licensed under the terms of the
  * GNU GPL, version 2 or (at your option) any later version.
  */
 
 #include "qemu/osdep.h"
 #include "block/block.h"
 #include "block/thread-pool.h"
 #include "qemu/main-loop.h"
 #include "qemu/rcu.h"
 #include "qemu/rcu_queue.h"
 #include "qemu/sockets.h"
 #include "qemu/cutils.h"
 #include "trace.h"
 #include "aio-posix.h"
 
 /* Stop userspace polling on a handler if it isn't active for some time */
 #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
 
 bool aio_poll_disabled(AioContext *ctx)
 {
     return qatomic_read(&ctx->poll_disable_cnt);
 }
 
 void aio_add_ready_handler(AioHandlerList *ready_list,
                            AioHandler *node,
                            int revents)
 {
     QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
     node->pfd.revents = revents;
     QLIST_INSERT_HEAD(ready_list, node, node_ready);
 }
 
 static void aio_add_poll_ready_handler(AioHandlerList *ready_list,
                                        AioHandler *node)
 {
     QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
     node->poll_ready = true;
     QLIST_INSERT_HEAD(ready_list, node, node_ready);
 }
 
 static AioHandler *find_aio_handler(AioContext *ctx, int fd)
 {
     AioHandler *node;
 
     QLIST_FOREACH(node, &ctx->aio_handlers, node) {
         if (node->pfd.fd == fd) {
             if (!QLIST_IS_INSERTED(node, node_deleted)) {
                 return node;
             }
         }
     }
 
     return NULL;
 }
 
 static bool aio_remove_fd_handler(AioContext *ctx, AioHandler *node)
 {
     /* If the GSource is in the process of being destroyed then
      * g_source_remove_poll() causes an assertion failure.  Skip
      * removal in that case, because glib cleans up its state during
      * destruction anyway.
      */
     if (!g_source_is_destroyed(&ctx->source)) {
         g_source_remove_poll(&ctx->source, &node->pfd);
     }
 
     node->pfd.revents = 0;
     node->poll_ready = false;
 
     /* If the fd monitor has already marked it deleted, leave it alone */
     if (QLIST_IS_INSERTED(node, node_deleted)) {
         return false;
     }
 
     /* If a read is in progress, just mark the node as deleted */
     if (qemu_lockcnt_count(&ctx->list_lock)) {
         QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
         return false;
     }
     /* Otherwise, delete it for real.  We can't just mark it as
      * deleted because deleted nodes are only cleaned up while
      * no one is walking the handlers list.
      */
     QLIST_SAFE_REMOVE(node, node_poll);
     QLIST_REMOVE(node, node);
     return true;
 }
 
 void aio_set_fd_handler(AioContext *ctx,
                         int fd,
                         bool is_external,
                         IOHandler *io_read,
                         IOHandler *io_write,
                         AioPollFn *io_poll,
                         IOHandler *io_poll_ready,
                         void *opaque)
 {
     AioHandler *node;
     AioHandler *new_node = NULL;
     bool is_new = false;
     bool deleted = false;
     int poll_disable_change;
 
     if (io_poll && !io_poll_ready) {
         io_poll = NULL; /* polling only makes sense if there is a handler */
     }
 
     qemu_lockcnt_lock(&ctx->list_lock);
 
     node = find_aio_handler(ctx, fd);
 
     /* Are we deleting the fd handler? */
     if (!io_read && !io_write && !io_poll) {
         if (node == NULL) {
             qemu_lockcnt_unlock(&ctx->list_lock);
             return;
         }
         /* Clean events in order to unregister fd from the ctx epoll. */
         node->pfd.events = 0;
 
         poll_disable_change = -!node->io_poll;
     } else {
         poll_disable_change = !io_poll - (node && !node->io_poll);
         if (node == NULL) {
             is_new = true;
         }
         /* Alloc and insert if it's not already there */
         new_node = g_new0(AioHandler, 1);
 
         /* Update handler with latest information */
         new_node->io_read = io_read;
         new_node->io_write = io_write;
         new_node->io_poll = io_poll;
         new_node->io_poll_ready = io_poll_ready;
         new_node->opaque = opaque;
         new_node->is_external = is_external;
 
         if (is_new) {
             new_node->pfd.fd = fd;
         } else {
             new_node->pfd = node->pfd;
         }
         g_source_add_poll(&ctx->source, &new_node->pfd);
 
         new_node->pfd.events = (io_read ? G_IO_IN | G_IO_HUP | G_IO_ERR : 0);
         new_node->pfd.events |= (io_write ? G_IO_OUT | G_IO_ERR : 0);
 
         QLIST_INSERT_HEAD_RCU(&ctx->aio_handlers, new_node, node);
     }
 
     /* No need to order poll_disable_cnt writes against other updates;
      * the counter is only used to avoid wasting time and latency on
      * iterated polling when the system call will be ultimately necessary.
      * Changing handlers is a rare event, and a little wasted polling until
      * the aio_notify below is not an issue.
      */
     qatomic_set(&ctx->poll_disable_cnt,
                qatomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
 
     ctx->fdmon_ops->update(ctx, node, new_node);
     if (node) {
         deleted = aio_remove_fd_handler(ctx, node);
     }
     qemu_lockcnt_unlock(&ctx->list_lock);
     aio_notify(ctx);
 
     if (deleted) {
         g_free(node);
     }
 }
 
 void aio_set_fd_poll(AioContext *ctx, int fd,
                      IOHandler *io_poll_begin,
                      IOHandler *io_poll_end)
 {
     AioHandler *node = find_aio_handler(ctx, fd);
 
     if (!node) {
         return;
     }
 
     node->io_poll_begin = io_poll_begin;
     node->io_poll_end = io_poll_end;
 }
 
 void aio_set_event_notifier(AioContext *ctx,
                             EventNotifier *notifier,
                             bool is_external,
                             EventNotifierHandler *io_read,
                             AioPollFn *io_poll,
                             EventNotifierHandler *io_poll_ready)
 {
     aio_set_fd_handler(ctx, event_notifier_get_fd(notifier), is_external,
                        (IOHandler *)io_read, NULL, io_poll,
                        (IOHandler *)io_poll_ready, notifier);
 }
 
 void aio_set_event_notifier_poll(AioContext *ctx,
                                  EventNotifier *notifier,
                                  EventNotifierHandler *io_poll_begin,
                                  EventNotifierHandler *io_poll_end)
 {
     aio_set_fd_poll(ctx, event_notifier_get_fd(notifier),
                     (IOHandler *)io_poll_begin,
                     (IOHandler *)io_poll_end);
 }
 
 static bool poll_set_started(AioContext *ctx, AioHandlerList *ready_list,
                              bool started)
 {
     AioHandler *node;
     bool progress = false;
 
     if (started == ctx->poll_started) {
         return false;
     }
 
     ctx->poll_started = started;
 
     qemu_lockcnt_inc(&ctx->list_lock);
     QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
         IOHandler *fn;
 
         if (QLIST_IS_INSERTED(node, node_deleted)) {
             continue;
         }
 
         if (started) {
             fn = node->io_poll_begin;
         } else {
             fn = node->io_poll_end;
         }
 
         if (fn) {
             fn(node->opaque);
         }
 
         /* Poll one last time in case ->io_poll_end() raced with the event */
         if (!started && node->io_poll(node->opaque)) {
             aio_add_poll_ready_handler(ready_list, node);
             progress = true;
         }
     }
     qemu_lockcnt_dec(&ctx->list_lock);
 
     return progress;
 }
 
 
 bool aio_prepare(AioContext *ctx)
 {
     AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
 
     /* Poll mode cannot be used with glib's event loop, disable it. */
     poll_set_started(ctx, &ready_list, false);
     /* TODO what to do with this list? */
 
     return false;
 }
 
 bool aio_pending(AioContext *ctx)
 {
     AioHandler *node;
     bool result = false;
 
     /*
      * We have to walk very carefully in case aio_set_fd_handler is
      * called while we're walking.
      */
     qemu_lockcnt_inc(&ctx->list_lock);
 
     QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
         int revents;
 
         /* TODO should this check poll ready? */
         revents = node->pfd.revents & node->pfd.events;
         if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
             aio_node_check(ctx, node->is_external)) {
             result = true;
             break;
         }
         if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write &&
             aio_node_check(ctx, node->is_external)) {
             result = true;
             break;
         }
     }
     qemu_lockcnt_dec(&ctx->list_lock);
 
     return result;
 }
 
 static void aio_free_deleted_handlers(AioContext *ctx)
 {
     AioHandler *node;
 
     if (QLIST_EMPTY_RCU(&ctx->deleted_aio_handlers)) {
         return;
     }
     if (!qemu_lockcnt_dec_if_lock(&ctx->list_lock)) {
         return; /* we are nested, let the parent do the freeing */
     }
 
     while ((node = QLIST_FIRST_RCU(&ctx->deleted_aio_handlers))) {
         QLIST_REMOVE(node, node);
         QLIST_REMOVE(node, node_deleted);
         QLIST_SAFE_REMOVE(node, node_poll);
         g_free(node);
     }
 
     qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
 }
 
 static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
 {
     bool progress = false;
     bool poll_ready;
     int revents;
 
     revents = node->pfd.revents & node->pfd.events;
     node->pfd.revents = 0;
 
     poll_ready = node->poll_ready;
     node->poll_ready = false;
 
     /*
      * Start polling AioHandlers when they become ready because activity is
      * likely to continue.  Note that starvation is theoretically possible when
      * fdmon_supports_polling(), but only until the fd fires for the first
      * time.
      */
     if (!QLIST_IS_INSERTED(node, node_deleted) &&
         !QLIST_IS_INSERTED(node, node_poll) &&
         node->io_poll) {
         trace_poll_add(ctx, node, node->pfd.fd, revents);
         if (ctx->poll_started && node->io_poll_begin) {
             node->io_poll_begin(node->opaque);
         }
         QLIST_INSERT_HEAD(&ctx->poll_aio_handlers, node, node_poll);
     }
     if (!QLIST_IS_INSERTED(node, node_deleted) &&
         poll_ready && revents == 0 &&
         aio_node_check(ctx, node->is_external) &&
         node->io_poll_ready) {
         node->io_poll_ready(node->opaque);
 
         /*
          * Return early since revents was zero. aio_notify() does not count as
          * progress.
          */
         return node->opaque != &ctx->notifier;
     }
 
     if (!QLIST_IS_INSERTED(node, node_deleted) &&
         (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) &&
         aio_node_check(ctx, node->is_external) &&
         node->io_read) {
         node->io_read(node->opaque);
 
         /* aio_notify() does not count as progress */
         if (node->opaque != &ctx->notifier) {
             progress = true;
         }
     }
     if (!QLIST_IS_INSERTED(node, node_deleted) &&
         (revents & (G_IO_OUT | G_IO_ERR)) &&
         aio_node_check(ctx, node->is_external) &&
         node->io_write) {
         node->io_write(node->opaque);
         progress = true;
     }
 
     return progress;
 }
 
 /*
  * If we have a list of ready handlers then this is more efficient than
  * scanning all handlers with aio_dispatch_handlers().
  */
 static bool aio_dispatch_ready_handlers(AioContext *ctx,
                                         AioHandlerList *ready_list)
 {
     bool progress = false;
     AioHandler *node;
 
     while ((node = QLIST_FIRST(ready_list))) {
         QLIST_REMOVE(node, node_ready);
         progress = aio_dispatch_handler(ctx, node) || progress;
     }
 
     return progress;
 }
 
 /* Slower than aio_dispatch_ready_handlers() but only used via glib */
 static bool aio_dispatch_handlers(AioContext *ctx)
 {
     AioHandler *node, *tmp;
     bool progress = false;
 
     QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
         progress = aio_dispatch_handler(ctx, node) || progress;
     }
 
     return progress;
 }
 
 void aio_dispatch(AioContext *ctx)
 {
     qemu_lockcnt_inc(&ctx->list_lock);
     aio_bh_poll(ctx);
     aio_dispatch_handlers(ctx);
     aio_free_deleted_handlers(ctx);
     qemu_lockcnt_dec(&ctx->list_lock);
 
     timerlistgroup_run_timers(&ctx->tlg);
 }
 
 static bool run_poll_handlers_once(AioContext *ctx,
                                    AioHandlerList *ready_list,
                                    int64_t now,
                                    int64_t *timeout)
 {
     bool progress = false;
     AioHandler *node;
     AioHandler *tmp;
 
     QLIST_FOREACH_SAFE(node, &ctx->poll_aio_handlers, node_poll, tmp) {
         if (aio_node_check(ctx, node->is_external) &&
             node->io_poll(node->opaque)) {
             aio_add_poll_ready_handler(ready_list, node);
 
             node->poll_idle_timeout = now + POLL_IDLE_INTERVAL_NS;
 
             /*
              * Polling was successful, exit try_poll_mode immediately
              * to adjust the next polling time.
              */
             *timeout = 0;
             if (node->opaque != &ctx->notifier) {
                 progress = true;
             }
         }
 
         /* Caller handles freeing deleted nodes.  Don't do it here. */
     }
 
     return progress;
 }
 
 static bool fdmon_supports_polling(AioContext *ctx)
 {
     return ctx->fdmon_ops->need_wait != aio_poll_disabled;
 }
 
 static bool remove_idle_poll_handlers(AioContext *ctx,
                                       AioHandlerList *ready_list,
                                       int64_t now)
 {
     AioHandler *node;
     AioHandler *tmp;
     bool progress = false;
 
     /*
      * File descriptor monitoring implementations without userspace polling
      * support suffer from starvation when a subset of handlers is polled
      * because fds will not be processed in a timely fashion.  Don't remove
      * idle poll handlers.
      */
     if (!fdmon_supports_polling(ctx)) {
         return false;
     }
 
     QLIST_FOREACH_SAFE(node, &ctx->poll_aio_handlers, node_poll, tmp) {
         if (node->poll_idle_timeout == 0LL) {
             node->poll_idle_timeout = now + POLL_IDLE_INTERVAL_NS;
         } else if (now >= node->poll_idle_timeout) {
             trace_poll_remove(ctx, node, node->pfd.fd);
             node->poll_idle_timeout = 0LL;
             QLIST_SAFE_REMOVE(node, node_poll);
             if (ctx->poll_started && node->io_poll_end) {
                 node->io_poll_end(node->opaque);
 
                 /*
                  * Final poll in case ->io_poll_end() races with an event.
                  * Nevermind about re-adding the handler in the rare case where
                  * this causes progress.
                  */
                 if (node->io_poll(node->opaque)) {
                     aio_add_poll_ready_handler(ready_list, node);
                     progress = true;
                 }
             }
         }
     }
 
     return progress;
 }
 
 /* run_poll_handlers:
  * @ctx: the AioContext
  * @ready_list: the list to place ready handlers on
  * @max_ns: maximum time to poll for, in nanoseconds
  *
  * Polls for a given time.
  *
  * Note that the caller must have incremented ctx->list_lock.
  *
  * Returns: true if progress was made, false otherwise
  */
 static bool run_poll_handlers(AioContext *ctx, AioHandlerList *ready_list,
                               int64_t max_ns, int64_t *timeout)
 {
     bool progress;
     int64_t start_time, elapsed_time;
 
     assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
 
     trace_run_poll_handlers_begin(ctx, max_ns, *timeout);
 
     /*
      * Optimization: ->io_poll() handlers often contain RCU read critical
      * sections and we therefore see many rcu_read_lock() -> rcu_read_unlock()
      * -> rcu_read_lock() -> ... sequences with expensive memory
      * synchronization primitives.  Make the entire polling loop an RCU
      * critical section because nested rcu_read_lock()/rcu_read_unlock() calls
      * are cheap.
      */
     RCU_READ_LOCK_GUARD();
 
     start_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     do {
         progress = run_poll_handlers_once(ctx, ready_list,
                                           start_time, timeout);
         elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
         max_ns = qemu_soonest_timeout(*timeout, max_ns);
         assert(!(max_ns && progress));
     } while (elapsed_time < max_ns && !ctx->fdmon_ops->need_wait(ctx));
 
     if (remove_idle_poll_handlers(ctx, ready_list,
                                   start_time + elapsed_time)) {
         *timeout = 0;
         progress = true;
     }
 
     /* If time has passed with no successful polling, adjust *timeout to
      * keep the same ending time.
      */
     if (*timeout != -1) {
         *timeout -= MIN(*timeout, elapsed_time);
     }
 
     trace_run_poll_handlers_end(ctx, progress, *timeout);
     return progress;
 }
 
 /* try_poll_mode:
  * @ctx: the AioContext
  * @ready_list: list to add handlers that need to be run
  * @timeout: timeout for blocking wait, computed by the caller and updated if
  *    polling succeeds.
  *
  * Note that the caller must have incremented ctx->list_lock.
  *
  * Returns: true if progress was made, false otherwise
  */
 static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
                           int64_t *timeout)
 {
     int64_t max_ns;
 
     if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
         return false;
     }
 
     max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
     if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
         poll_set_started(ctx, ready_list, true);
 
         if (run_poll_handlers(ctx, ready_list, max_ns, timeout)) {
             return true;
         }
     }
 
     if (poll_set_started(ctx, ready_list, false)) {
         *timeout = 0;
         return true;
     }
 
     return false;
 }
 
 bool aio_poll(AioContext *ctx, bool blocking)
 {
     AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
     bool progress;
     bool use_notify_me;
     int64_t timeout;
     int64_t start = 0;
 
     /*
      * There cannot be two concurrent aio_poll calls for the same AioContext (or
      * an aio_poll concurrent with a GSource prepare/check/dispatch callback).
      * We rely on this below to avoid slow locked accesses to ctx->notify_me.
      *
      * aio_poll() may only be called in the AioContext's thread. iohandler_ctx
      * is special in that it runs in the main thread, but that thread's context
      * is qemu_aio_context.
      */
     assert(in_aio_context_home_thread(ctx == iohandler_get_aio_context() ?
                                       qemu_get_aio_context() : ctx));
 
     qemu_lockcnt_inc(&ctx->list_lock);
 
     if (ctx->poll_max_ns) {
         start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     }
 
     timeout = blocking ? aio_compute_timeout(ctx) : 0;
     progress = try_poll_mode(ctx, &ready_list, &timeout);
     assert(!(timeout && progress));
 
     /*
      * aio_notify can avoid the expensive event_notifier_set if
      * everything (file descriptors, bottom halves, timers) will
      * be re-evaluated before the next blocking poll().  This is
      * already true when aio_poll is called with blocking == false;
      * if blocking == true, it is only true after poll() returns,
      * so disable the optimization now.
      */
     use_notify_me = timeout != 0;
     if (use_notify_me) {
         qatomic_set(&ctx->notify_me, qatomic_read(&ctx->notify_me) + 2);
         /*
          * Write ctx->notify_me before reading ctx->notified.  Pairs with
          * smp_mb in aio_notify().
          */
         smp_mb();
 
         /* Don't block if aio_notify() was called */
         if (qatomic_read(&ctx->notified)) {
             timeout = 0;
         }
     }
 
     /* If polling is allowed, non-blocking aio_poll does not need the
      * system call---a single round of run_poll_handlers_once suffices.
      */
     if (timeout || ctx->fdmon_ops->need_wait(ctx)) {
         ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
     }
 
     if (use_notify_me) {
         /* Finish the poll before clearing the flag.  */
         qatomic_store_release(&ctx->notify_me,
                              qatomic_read(&ctx->notify_me) - 2);
     }
 
     aio_notify_accept(ctx);
 
     /* Adjust polling time */
     if (ctx->poll_max_ns) {
         int64_t block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start;
 
         if (block_ns <= ctx->poll_ns) {
             /* This is the sweet spot, no adjustment needed */
         } else if (block_ns > ctx->poll_max_ns) {
             /* We'd have to poll for too long, poll less */
             int64_t old = ctx->poll_ns;
 
             if (ctx->poll_shrink) {
                 ctx->poll_ns /= ctx->poll_shrink;
             } else {
                 ctx->poll_ns = 0;
             }
 
             trace_poll_shrink(ctx, old, ctx->poll_ns);
         } else if (ctx->poll_ns < ctx->poll_max_ns &&
                    block_ns < ctx->poll_max_ns) {
             /* There is room to grow, poll longer */
             int64_t old = ctx->poll_ns;
             int64_t grow = ctx->poll_grow;
 
             if (grow == 0) {
                 grow = 2;
             }
 
             if (ctx->poll_ns) {
                 ctx->poll_ns *= grow;
             } else {
                 ctx->poll_ns = 4000; /* start polling at 4 microseconds */
             }
 
             if (ctx->poll_ns > ctx->poll_max_ns) {
                 ctx->poll_ns = ctx->poll_max_ns;
             }
 
             trace_poll_grow(ctx, old, ctx->poll_ns);
         }
     }
 
     progress |= aio_bh_poll(ctx);
     progress |= aio_dispatch_ready_handlers(ctx, &ready_list);
 
     aio_free_deleted_handlers(ctx);
 
     qemu_lockcnt_dec(&ctx->list_lock);
 
     progress |= timerlistgroup_run_timers(&ctx->tlg);
 
     return progress;
 }
 
 void aio_context_setup(AioContext *ctx)
 {
     ctx->fdmon_ops = &fdmon_poll_ops;
     ctx->epollfd = -1;
 
     /* Use the fastest fd monitoring implementation if available */
     if (fdmon_io_uring_setup(ctx)) {
         return;
     }
 
     fdmon_epoll_setup(ctx);
 }
 
 void aio_context_destroy(AioContext *ctx)
 {
     fdmon_io_uring_destroy(ctx);
     fdmon_epoll_disable(ctx);
     aio_free_deleted_handlers(ctx);
 }
 
 void aio_context_use_g_source(AioContext *ctx)
 {
     /*
      * Disable io_uring when the glib main loop is used because it doesn't
      * support mixed glib/aio_poll() usage. It relies on aio_poll() being
      * called regularly so that changes to the monitored file descriptors are
      * submitted, otherwise a list of pending fd handlers builds up.
      */
     fdmon_io_uring_destroy(ctx);
     aio_free_deleted_handlers(ctx);
 }
 
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
                                  int64_t grow, int64_t shrink, Error **errp)
 {
     /* No thread synchronization here, it doesn't matter if an incorrect value
      * is used once.
      */
     ctx->poll_max_ns = max_ns;
     ctx->poll_ns = 0;
     ctx->poll_grow = grow;
     ctx->poll_shrink = shrink;
 
     aio_notify(ctx);
 }
 
 void aio_context_set_aio_params(AioContext *ctx, int64_t max_batch,
                                 Error **errp)
 {
     /*
      * No thread synchronization here, it doesn't matter if an incorrect value
      * is used once.
      */
     ctx->aio_max_batch = max_batch;
 
     aio_notify(ctx);
 }