[RFC PATCH 06/12] io_uring: add IO_URING_F_FUSED and prepare for supporting OP_FUSED_CMD

[Ming Lei <ming.lei@redhat.com>]

Add flag IO_URING_F_FUSED and prepare for supporting IO_URING_OP_FUSED_CMD,
which is still one type of IO_URING_OP_URING_CMD, so it is reasonable
to reuse ->uring_cmd() for handling IO_URING_F_FUSED_CMD.

And just IO_URING_F_FUSED_CMD will carry one 64byte SQE as payload which
will be handled by one slave request. The master uring command will
provide kernel buffer to the slave request via 'struct io_mapped_buf'.

Mark all existed drivers to not support IO_URING_F_FUSED_CMD, given it
depends if driver is capable of handling the slave request.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/ublk_drv.c  | 6 ++++++
 drivers/char/mem.c        | 4 ++++
 drivers/nvme/host/ioctl.c | 9 +++++++++
 include/linux/io_uring.h  | 7 +++++++
 4 files changed, 26 insertions(+)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index b9c759cef00e..c89ede1c9b22 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -1274,6 +1274,9 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 	if (!(issue_flags & IO_URING_F_SQE128))
 		goto out;
 
+	if (issue_flags & IO_URING_F_FUSED)
+		return -EOPNOTSUPP;
+
 	if (ub_cmd->q_id >= ub->dev_info.nr_hw_queues)
 		goto out;
 
@@ -2172,6 +2175,9 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 	struct ublk_device *ub = NULL;
 	int ret = -EINVAL;
 
+	if (issue_flags & IO_URING_F_FUSED)
+		return -EOPNOTSUPP;
+
 	if (issue_flags & IO_URING_F_NONBLOCK)
 		return -EAGAIN;
 
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index ffb101d349f0..134ba6665194 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -30,6 +30,7 @@
 #include <linux/uio.h>
 #include <linux/uaccess.h>
 #include <linux/security.h>
+#include <linux/io_uring.h>
 
 #ifdef CONFIG_IA64
 # include <linux/efi.h>
@@ -482,6 +483,9 @@ static ssize_t splice_write_null(struct pipe_inode_info *pipe, struct file *out,
 
 static int uring_cmd_null(struct io_uring_cmd *ioucmd, unsigned int issue_flags)
 {
+	if (issue_flags & IO_URING_F_FUSED)
+		return -EOPNOTSUPP;
+
 	return 0;
 }
 
diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
index 723e7d5b778f..44a171bcaa90 100644
--- a/drivers/nvme/host/ioctl.c
+++ b/drivers/nvme/host/ioctl.c
@@ -773,6 +773,9 @@ int nvme_ns_chr_uring_cmd(struct io_uring_cmd *ioucmd, unsigned int issue_flags)
 	struct nvme_ns *ns = container_of(file_inode(ioucmd->file)->i_cdev,
 			struct nvme_ns, cdev);
 
+	if (issue_flags & IO_URING_F_FUSED)
+		return -EOPNOTSUPP;
+
 	return nvme_ns_uring_cmd(ns, ioucmd, issue_flags);
 }
 
@@ -878,6 +881,9 @@ int nvme_ns_head_chr_uring_cmd(struct io_uring_cmd *ioucmd,
 	struct nvme_ns *ns = nvme_find_path(head);
 	int ret = -EINVAL;
 
+	if (issue_flags & IO_URING_F_FUSED)
+		return -EOPNOTSUPP;
+
 	if (ns)
 		ret = nvme_ns_uring_cmd(ns, ioucmd, issue_flags);
 	srcu_read_unlock(&head->srcu, srcu_idx);
@@ -915,6 +921,9 @@ int nvme_dev_uring_cmd(struct io_uring_cmd *ioucmd, unsigned int issue_flags)
 	struct nvme_ctrl *ctrl = ioucmd->file->private_data;
 	int ret;
 
+	if (issue_flags & IO_URING_F_FUSED)
+		return -EOPNOTSUPP;
+
 	/* IOPOLL not supported yet */
 	if (issue_flags & IO_URING_F_IOPOLL)
 		return -EOPNOTSUPP;
diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
index 88205ea566d3..2ccf91146c13 100644
--- a/include/linux/io_uring.h
+++ b/include/linux/io_uring.h
@@ -21,6 +21,13 @@ enum io_uring_cmd_flags {
 	IO_URING_F_SQE128		= (1 << 8),
 	IO_URING_F_CQE32		= (1 << 9),
 	IO_URING_F_IOPOLL		= (1 << 10),
+
+	/* for FUSED_CMD only */
+	IO_URING_F_FUSED_WRITE		= (1 << 11), /* slave writes to buffer */
+	IO_URING_F_FUSED_READ		= (1 << 12), /* slave reads from buffer */
+	/* driver incapable of FUSED_CMD should fail cmd when seeing F_FUSED */
+	IO_URING_F_FUSED		= IO_URING_F_FUSED_WRITE |
+		IO_URING_F_FUSED_READ,
 };
 
 struct io_uring_cmd {
-- 
2.31.1