[PATCH v6 0/3] bsg: add io_uring command support for SCSI passthrough

[Yang Xiuwei <yangxiuwei@kylinos.cn>]

This series adds io_uring command support to the BSG SCSI passthrough
path.

The goal is to allow userspace to submit SCSI passthrough commands via
IORING_OP_URING_CMD, in addition to the existing sg_io interface.
The io_uring path mirrors the existing BSG behaviour: it currently only
supports BSG_PROTOCOL_SCSI + BSG_SUB_PROTOCOL_SCSI_CMD and does not
support BIDI transfers.

Patch 1 defines struct bsg_uring_cmd in the UAPI so that userspace can
describe SCSI passthrough requests for io_uring.

Patch 2 extends the generic BSG layer with an .uring_cmd file operation
and a bsg_uring_cmd_fn callback, allowing transport-specific handlers to
be registered.

Patch 3 implements the SCSI BSG io_uring handler. It builds a SCSI
request from struct bsg_uring_cmd, maps user buffers (including fixed
buffers), and completes asynchronously via a request end_io callback and
task_work. Completion returns SCSI device/host/driver status, residual
length and sense data length packed into the CQE result.

Changes since v5:
- Fixed compilation errors:
  * Use io_uring_sqe128_cmd() instead of io_uring_sqe_cmd()
  * Update scsi_bsg_uring_cmd_done() signature to match rq_end_io_fn typedef

Compared to the earlier RFC v4 series [1], this version only includes
minor code cleanups (mainly comments and wording), without changing the
behaviour or logic of the implementation.

[1] https://lore.kernel.org/linux-block/20260122015653.703188-1-yangxiuwei@kylinos.cn/

Testing
-------
Testing was done inside a VM on a disk with:
  /sys/block/sdd/mq/0/nr_tags     = 1024
  /sys/block/sdd/queue/nr_requests = 256

The following SCSI INQUIRY micro-benchmark was run with N=100000:

  sg+SG_IO (v3), /dev/sg4:
    avg = 139.0 us, p50 = 128.9 us, p90 = 149.7 us, p99 = 301.0 us

  bsg+SG_IO (v4), /dev/bsg/2:0:0:0:
    avg = 97.2 us,  p50 = 92.7 us,  p90 = 111.5 us, p99 = 150.9 us

  bsg+io_uring, /dev/bsg/2:0:0:0:
    avg = 105.9 us, p50 = 95.4 us,  p90 = 116.2 us, p99 = 175.0 us

  bsg+io_uring (batch=64), /dev/bsg/2:0:0:0:
    avg = 61.9 us,  p50 = 60.9 us,  p90 = 63.9 us,  p99 = 94.6 us

These results show that the new io_uring path is comparable to the
existing BSG SG_IO interface for single-command workloads, and that
batched io_uring submission can significantly improve latency for
high-concurrency workloads when the device supports sufficient queue
depth.

Yang Xiuwei (3):
  bsg: add bsg_uring_cmd uapi structure
  bsg: add io_uring command support to generic layer
  scsi: bsg: add io_uring passthrough handler

 block/bsg-lib.c          |   2 +-
 block/bsg.c              |  32 +++++-
 drivers/scsi/scsi_bsg.c  | 207 ++++++++++++++++++++++++++++++++++++++-
 include/linux/bsg.h      |   6 +-
 include/uapi/linux/bsg.h |  21 ++++
 5 files changed, 264 insertions(+), 4 deletions(-)

base-commit: af4e9ef3d78420feb8fe58cd9a1ab80c501b3c08
-- 
2.25.1