io_uring/query: introduce rings info query

Same problem as with zcrx in the previous patch, the user needs to know
SQ/CQ header sizes to allocated memory before setup to use it for user
provided rings, i.e. IORING_SETUP_NO_MMAP, however that information is
only returned after registration, hence the user is guessing kernel
implementation details.

Return the header size and alignment, which is split with the same
motivation, to allow the user to know the real structure size without
alignment in case there will be more flexible placement schemes in the
future.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

include/uapi/linux/io_uring/query.h

@@ -19,6 +19,7 @@ struct io_uring_query_hdr {
 enum {
 	IO_URING_QUERY_OPCODES			= 0,
 	IO_URING_QUERY_ZCRX			= 1,
+	IO_URING_QUERY_SCQ			= 2,
 
 	__IO_URING_QUERY_MAX,
 };
@@ -57,4 +58,11 @@ struct io_uring_query_zcrx {
 	__u64 __resv2;
 };
 
+struct io_uring_query_scq {
+	/* The SQ/CQ rings header size */
+	__u64 hdr_size;
+	/* The alignment for the header */
+	__u64 hdr_alignment;
+};
+
 #endif

io_uring/query.c

@@ -9,6 +9,7 @@
 union io_query_data {
 	struct io_uring_query_opcode opcodes;
 	struct io_uring_query_zcrx zcrx;
+	struct io_uring_query_scq scq;
 };
 
 #define IO_MAX_QUERY_SIZE		sizeof(union io_query_data)
@@ -43,6 +44,15 @@ static ssize_t io_query_zcrx(union io_query_data *data)
 	return sizeof(*e);
 }
 
+static ssize_t io_query_scq(union io_query_data *data)
+{
+	struct io_uring_query_scq *e = &data->scq;
+
+	e->hdr_size = sizeof(struct io_rings);
+	e->hdr_alignment = SMP_CACHE_BYTES;
+	return sizeof(*e);
+}
+
 static int io_handle_query_entry(struct io_ring_ctx *ctx,
 				 union io_query_data *data, void __user *uhdr,
 				 u64 *next_entry)
@@ -74,6 +84,9 @@ static int io_handle_query_entry(struct io_ring_ctx *ctx,
 	case IO_URING_QUERY_ZCRX:
 		ret = io_query_zcrx(data);
 		break;
+	case IO_URING_QUERY_SCQ:
+		ret = io_query_scq(data);
+		break;
 	}
 
 	if (ret >= 0) {