io_uring: add IORING_CQ_EVENTFD_DISABLED to the CQ ring flags

This new flag should be set/clear from the application to
disable/enable eventfd notifications when a request is completed
and queued to the CQ ring.

Before this patch, notifications were always sent if an eventfd is
registered, so IORING_CQ_EVENTFD_DISABLED is not set during the
initialization.

It will be up to the application to set the flag after initialization
if no notifications are required at the beginning.
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1153,6 +1153,8 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
 {
 	if (!ctx->cq_ev_fd)
 		return false;
+	if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
+		return false;
 	if (!ctx->eventfd_async)
 		return true;
 	return io_wq_current_is_worker();

include/uapi/linux/io_uring.h

@@ -209,6 +209,13 @@ struct io_cqring_offsets {
 	__u64 resv2;
 };
 
+/*
+ * cq_ring->flags
+ */
+
+/* disable eventfd notifications */
+#define IORING_CQ_EVENTFD_DISABLED	(1U << 0)
+
 /*
  * io_uring_enter(2) flags
  */