aio-posix: keep polling enabled with fdmon-io_uring.c

Commit 816a430c51 ("util/aio: Defer disabling poll mode as long as
possible") kept polling enabled when the event loop timeout is 0. Since
there is no timeout the event loop will continue immediately and the
overhead of disabling and re-enabling polling can be avoided.

fdmon-io_uring.c is unable to take advantage of this optimization
because its ->need_wait() function returns true whenever there are new
io_uring SQEs to submit:

  if (timeout || ctx->fdmon_ops->need_wait(ctx)) {
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Polling will be disabled even when timeout == 0.

Extend the optimization to handle the case when need_wait() returns true
and timeout == 0.

Cc: Chao Gao <chao.gao@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20251104022933.618123-5-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

util/aio-posix.c

@@ -559,7 +559,14 @@ static bool run_poll_handlers(AioContext *ctx, AioHandlerList *ready_list,
         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 (ctx->fdmon_ops->need_wait(ctx)) {
+            if (fdmon_supports_polling(ctx)) {
+                *timeout = 0; /* stay in polling mode */
+            }
+            break;
+        }
+    } while (elapsed_time < max_ns);
 
     if (remove_idle_poll_handlers(ctx, ready_list,
                                   start_time + elapsed_time)) {
@@ -722,7 +729,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
          * up IO threads when some work becomes pending. It is essential to
          * avoid hangs or unnecessary latency.
          */
-        if (poll_set_started(ctx, &ready_list, false)) {
+        if (timeout && poll_set_started(ctx, &ready_list, false)) {
             timeout = 0;
             progress = true;
         }