io_uring: add 'cq_flags' field for the CQ ring

This patch adds the new 'cq_flags' field that should be written by
the application and read by the kernel.

This new field is available to the userspace application through
'cq_off.flags'.
We are using 4-bytes previously reserved and set to zero. This means
that if the application finds this field to zero, then the new
functionality is not supported.

In the next patch we will introduce the first flag available.
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -142,7 +142,7 @@ struct io_rings {
 	 */
 	u32			sq_dropped;
 	/*
-	 * Runtime flags
+	 * Runtime SQ flags
 	 *
 	 * Written by the kernel, shouldn't be modified by the
 	 * application.
@@ -151,6 +151,13 @@ struct io_rings {
 	 * for IORING_SQ_NEED_WAKEUP after updating the sq tail.
 	 */
 	u32			sq_flags;
+	/*
+	 * Runtime CQ flags
+	 *
+	 * Written by the application, shouldn't be modified by the
+	 * kernel.
+	 */
+	u32                     cq_flags;
 	/*
 	 * Number of completion events lost because the queue was full;
 	 * this should be avoided by the application by making sure
@@ -7930,6 +7937,7 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
 	p->cq_off.ring_entries = offsetof(struct io_rings, cq_ring_entries);
 	p->cq_off.overflow = offsetof(struct io_rings, cq_overflow);
 	p->cq_off.cqes = offsetof(struct io_rings, cqes);
+	p->cq_off.flags = offsetof(struct io_rings, cq_flags);
 
 	p->features = IORING_FEAT_SINGLE_MMAP | IORING_FEAT_NODROP |
 			IORING_FEAT_SUBMIT_STABLE | IORING_FEAT_RW_CUR_POS |

include/uapi/linux/io_uring.h

@@ -204,7 +204,9 @@ struct io_cqring_offsets {
 	__u32 ring_entries;
 	__u32 overflow;
 	__u32 cqes;
-	__u64 resv[2];
+	__u32 flags;
+	__u32 resv1;
+	__u64 resv2;
 };
 
 /*