[RFC PATCH liburing 1/2] Add support IORING_SETUP_SQE_MIXED

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

This adds core support for mixed sized SQEs in the same SQ ring. Before
this, SQEs were either 64b in size (the normal size), or 128b if
IORING_SETUP_SQE128 was set in the ring initialization. With the mixed
support, an SQE may be either 64b or 128b on the same SQ ring. If the
SQE is 128b in size, then IOSQE_SQE_128B will be set in the sqe flags.
The client may post a NOP SQE with IOSQE_CQE_SKIP_SUCCESS set that the
kernel should simply ignore as it's just a pad filler that is posted
when required.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
 src/include/liburing.h          | 31 +++++++++++++++++++++++++++++++
 src/include/liburing/io_uring.h |  9 +++++++++
 2 files changed, 40 insertions(+)

diff --git a/src/include/liburing.h b/src/include/liburing.h
index 7ea876e1..dc94ad13 100644
--- a/src/include/liburing.h
+++ b/src/include/liburing.h
@@ -1853,6 +1853,37 @@ IOURINGINLINE struct io_uring_sqe *_io_uring_get_sqe(struct io_uring *ring)
 	return sqe;
 }
 
+IOURINGINLINE struct io_uring_sqe *_io_uring_get_sqe128_mixed(struct io_uring *ring)
+	LIBURING_NOEXCEPT
+{
+	struct io_uring_sq *sq = &ring->sq;
+	unsigned head = io_uring_load_sq_head(ring), tail = sq->sqe_tail;
+	struct io_uring_sqe *sqe;
+
+	if (!(ring->flags & IORING_SETUP_SQE_MIXED))
+		return NULL;
+
+	if ((tail & sq->ring_mask) + 1 == sq->ring_entries) {
+		sqe = _io_uring_get_sqe(ring);
+		if (!sqe)
+			return NULL;
+
+		io_uring_prep_nop(sqe);
+		sqe->flags |= IOSQE_CQE_SKIP_SUCCESS;
+		tail = sq->sqe_tail;
+	}
+
+	if ((tail + 1) - head >= sq->ring_entries)
+		return NULL;
+
+	sqe = &sq->sqes[tail & sq->ring_mask];
+	sq->sqe_tail = tail + 2;
+	io_uring_initialize_sqe(sqe);
+	sqe->flags |= IOSQE_SQE_128B;
+
+	return sqe;
+}
+
 /*
  * Return the appropriate mask for a buffer ring of size 'ring_entries'
  */
diff --git a/src/include/liburing/io_uring.h b/src/include/liburing/io_uring.h
index 643514e5..fd02fa52 100644
--- a/src/include/liburing/io_uring.h
+++ b/src/include/liburing/io_uring.h
@@ -126,6 +126,7 @@ enum io_uring_sqe_flags_bit {
 	IOSQE_ASYNC_BIT,
 	IOSQE_BUFFER_SELECT_BIT,
 	IOSQE_CQE_SKIP_SUCCESS_BIT,
+	IOSQE_SQE_128B_BIT,
 };
 
 /*
@@ -145,6 +146,8 @@ enum io_uring_sqe_flags_bit {
 #define IOSQE_BUFFER_SELECT	(1U << IOSQE_BUFFER_SELECT_BIT)
 /* don't post CQE if request succeeded */
 #define IOSQE_CQE_SKIP_SUCCESS	(1U << IOSQE_CQE_SKIP_SUCCESS_BIT)
+/* this is a 128b/big-sqe posting */
+#define IOSQE_SQE_128B          (1U << IOSQE_SQE_128B_BIT)
 
 /*
  * io_uring_setup() flags
@@ -211,6 +214,12 @@ enum io_uring_sqe_flags_bit {
  */
 #define IORING_SETUP_CQE_MIXED		(1U << 18)
 
+/*
+ *  Allow both 64b and 128b SQEs. If a 128b SQE is posted, it will have
+ *  IOSQE_SQE_128B set in sqe->flags.
+ */
+#define IORING_SETUP_SQE_MIXED		(1U << 19)
+
 enum io_uring_op {
 	IORING_OP_NOP,
 	IORING_OP_READV,
-- 
2.47.3