[RFC PATCH v2 0/3] bsg: add io_uring command support for SCSI passthrough

[Yang Xiuwei <yangxiuwei@kylinos.cn>]

This RFC series adds io_uring command support to the BSG (Block layer
SCSI Generic) driver, enabling asynchronous SCSI passthrough operations
via io_uring.

Motivation:
-----------
The current BSG interface uses ioctl() for SCSI passthrough, which is
synchronous and has limitations for high-performance applications. By
integrating with io_uring, we can provide:

1. Asynchronous I/O support for better scalability
2. Zero-copy I/O via io_uring fixed buffers
3. Better integration with modern async I/O frameworks
4. Reduced system call overhead

Design:
-------
The implementation follows the io_uring uring_cmd pattern used by other
drivers (e.g., nvme). Key design decisions:

1. UAPI Structure: A new bsg_uring_cmd structure is defined that fits
   within the 80-byte cmd field of a 128-byte SQE, with 24 bytes reserved
   for future extensions. The structure uses protocol-agnostic field names
   to support multiple protocols beyond SCSI.

2. Status Information: SCSI status (device_status, host_status,
   driver_status, sense_len, resid_len) is returned in the CQE res2 field
   using a compact 64-bit encoding.

3. Zero-copy Support: The implementation supports both traditional
   user buffers and io_uring fixed buffers for zero-copy I/O.

4. Async Completion: Command completion is handled via task work to
   safely access user space and copy sense data.

5. Non-blocking I/O: Support for IO_URING_F_NONBLOCK flag to enable
   non-blocking command submission.

Limitations:
-----------
- Currently only SCSI commands are supported (BSG_PROTOCOL_SCSI)
- Scatter/gather I/O (iovec arrays) is not currently supported, but
  the data structure includes fields for future implementation.
- Bidirectional transfers are not supported (consistent with existing
  BSG behavior).

Testing:
--------
A user-space test program has been developed to validate the
implementation, including:
- Basic SCSI commands (INQUIRY, READ CAPACITY (10), READ (10),
  WRITE (10))
- Zero-copy mode using fixed buffers
- Error handling (invalid flags, unsupported features)

The test program is available separately and can be provided upon request.

Changes since v1:
-----------------
- Renamed SCSI-specific fields (cdb_addr/cdb_len) to protocol-agnostic
  names (request/request_len) to support multiple protocols beyond SCSI
- Removed __packed attribute and optimized field alignment to avoid
  suboptimal code generation on architectures that don't support unaligned
  accesses
- Simplified data transfer structure: unified din_xferp/dout_xferp into a
  single xfer_addr field with xfer_dir to indicate direction (0=read, 1=write),
  consistent with existing BSG behavior where bidirectional transfers are not
  supported
- Updated implementation to use new protocol-agnostic field names

Yang Xiuwei (3):
  bsg: add bsg_uring_cmd uapi structure
  bsg: add uring_cmd support to BSG generic layer
  bsg: implement SCSI BSG uring_cmd handler

 block/bsg.c              |  28 +++++
 drivers/scsi/scsi_bsg.c  | 222 +++++++++++++++++++++++++++++++++++++++
 include/linux/bsg.h      |   4 +
 include/uapi/linux/bsg.h |  19 ++++
 4 files changed, 273 insertions(+)

-- 
2.25.1