ublk: support UBLK_AUTO_BUF_REG_FALLBACK

For UBLK_F_AUTO_BUF_REG, buffer is registered to uring_cmd context
automatically with the provided buffer index. User may provide one wrong
buffer index, or the specified buffer is registered by application already.

Add UBLK_AUTO_BUF_REG_FALLBACK for supporting to auto buffer registering
fallback by completing the uring_cmd and telling ublk server the
register failure via UBLK_AUTO_BUF_REG_FALLBACK, then ublk server still
can register the buffer from userspace.

So we can provide reliable way for supporting auto buffer register.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250520045455.515691-5-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

2 files changed, 52 insertions, 3 deletions

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 1aabc655652b..2474788ef263 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -1182,6 +1182,16 @@ static inline void __ublk_abort_rq(struct ublk_queue *ubq,
 		blk_mq_end_request(rq, BLK_STS_IOERR);
 }
 
+static void ublk_auto_buf_reg_fallback(struct request *req, struct ublk_io *io)
+{
+	const struct ublk_queue *ubq = req->mq_hctx->driver_data;
+	struct ublksrv_io_desc *iod = ublk_get_iod(ubq, req->tag);
+	struct ublk_rq_data *data = blk_mq_rq_to_pdu(req);
+
+	iod->op_flags |= UBLK_IO_F_NEED_REG_BUF;
+	refcount_set(&data->ref, 1);
+}
+
 static bool ublk_auto_buf_reg(struct request *req, struct ublk_io *io,
 			      unsigned int issue_flags)
 {
@@ -1192,6 +1202,10 @@ static bool ublk_auto_buf_reg(struct request *req, struct ublk_io *io,
 	ret = io_buffer_register_bvec(io->cmd, req, ublk_io_release,
 				      pdu->buf.index, issue_flags);
 	if (ret) {
+		if (pdu->buf.flags & UBLK_AUTO_BUF_REG_FALLBACK) {
+			ublk_auto_buf_reg_fallback(req, io);
+			return true;
+		}
 		blk_mq_end_request(req, BLK_STS_IOERR);
 		return false;
 	}
@@ -1971,6 +1985,8 @@ static inline int ublk_set_auto_buf_reg(struct io_uring_cmd *cmd)
 	if (pdu->buf.reserved0 || pdu->buf.reserved1)
 		return -EINVAL;
 
+	if (pdu->buf.flags & ~UBLK_AUTO_BUF_REG_F_MASK)
+		return -EINVAL;
 	return 0;
 }
 
diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
index f6f516b1223b..c4b9942697fc 100644
--- a/include/uapi/linux/ublk_cmd.h
+++ b/include/uapi/linux/ublk_cmd.h
@@ -236,9 +236,16 @@
  *
  * - all reserved fields in `ublk_auto_buf_reg` need to be zeroed
  *
+ * - pass flags from `ublk_auto_buf_reg.flags` if needed
+ *
  * This way avoids extra cost from two uring_cmd, but also simplifies backend
  * implementation, such as, the dependency on IO_REGISTER_IO_BUF and
  * IO_UNREGISTER_IO_BUF becomes not necessary.
+ *
+ * If wrong data or flags are provided, both IO_FETCH_REQ and
+ * IO_COMMIT_AND_FETCH_REQ are failed, for the latter, the ublk IO request
+ * won't be completed until new IO_COMMIT_AND_FETCH_REQ command is issued
+ * successfully
  */
 #define UBLK_F_AUTO_BUF_REG 	(1ULL << 11)
 
@@ -328,6 +335,17 @@ struct ublksrv_ctrl_dev_info {
 #define		UBLK_IO_F_FUA			(1U << 13)
 #define		UBLK_IO_F_NOUNMAP		(1U << 15)
 #define		UBLK_IO_F_SWAP			(1U << 16)
+/*
+ * For UBLK_F_AUTO_BUF_REG & UBLK_AUTO_BUF_REG_FALLBACK only.
+ *
+ * This flag is set if auto buffer register is failed & ublk server passes
+ * UBLK_AUTO_BUF_REG_FALLBACK, and ublk server need to register buffer
+ * manually for handling the delivered IO command if this flag is observed
+ *
+ * ublk server has to check this flag if UBLK_AUTO_BUF_REG_FALLBACK is
+ * passed in.
+ */
+#define		UBLK_IO_F_NEED_REG_BUF		(1U << 17)
 
 /*
  * io cmd is described by this structure, and stored in share memory, indexed
@@ -362,10 +380,23 @@ static inline __u32 ublksrv_get_flags(const struct ublksrv_io_desc *iod)
 	return iod->op_flags >> 8;
 }
 
+/*
+ * If this flag is set, fallback by completing the uring_cmd and setting
+ * `UBLK_IO_F_NEED_REG_BUF` in case of auto-buf-register failure;
+ * otherwise the client ublk request is failed silently
+ *
+ * If ublk server passes this flag, it has to check if UBLK_IO_F_NEED_REG_BUF
+ * is set in `ublksrv_io_desc.op_flags`. If UBLK_IO_F_NEED_REG_BUF is set,
+ * ublk server needs to register io buffer manually for handling IO command.
+ */
+#define UBLK_AUTO_BUF_REG_FALLBACK 	(1 << 0)
+#define UBLK_AUTO_BUF_REG_F_MASK 	UBLK_AUTO_BUF_REG_FALLBACK
+
 struct ublk_auto_buf_reg {
 	/* index for registering the delivered request buffer */
 	__u16  index;
-	__u16   reserved0;
+	__u8   flags;
+	__u8   reserved0;
 
 	/*
 	 * io_ring FD can be passed via the reserve field in future for
@@ -379,6 +410,7 @@ struct ublk_auto_buf_reg {
  * uring_cmd's sqe->addr:
  *
  * 	- bit0 ~ bit15: buffer index
+ * 	- bit16 ~ bit23: flags
  * 	- bit24 ~ bit31: reserved0
  * 	- bit32 ~ bit63: reserved1
  */
@@ -387,7 +419,8 @@ static inline struct ublk_auto_buf_reg ublk_sqe_addr_to_auto_buf_reg(
 {
 	struct ublk_auto_buf_reg reg = {
 		.index = sqe_addr & 0xffff,
-		.reserved0 = (sqe_addr >> 16) & 0xffff,
+		.flags = (sqe_addr >> 16) & 0xff,
+		.reserved0 = (sqe_addr >> 24) & 0xff,
 		.reserved1 = sqe_addr >> 32,
 	};
 
@@ -397,7 +430,7 @@ static inline struct ublk_auto_buf_reg ublk_sqe_addr_to_auto_buf_reg(
 static inline __u64
 ublk_auto_buf_reg_to_sqe_addr(const struct ublk_auto_buf_reg *buf)
 {
-	__u64 addr = buf->index | (__u64)buf->reserved0 << 16 |
+	__u64 addr = buf->index | (__u64)buf->flags << 16 | (__u64)buf->reserved0 << 24 |
 		(__u64)buf->reserved1 << 32;
 
 	return addr;