[RFC PATCH 0/6] Fixed-buffers io_uring passthrough over nvme-char

[Kanchan Joshi <joshi.k@samsung.com>]

Hi,

This series takes a dig at expanding io_uring passthrough to have
fixed-buffer support.
For the NVMe side plumbing, it uses the per-namespace char-device as
the backend.

The work is on top of  Jens' branch (which is 5.14-rc1 based):
https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-fops.v5

Testing is done by integrating user-space side with fio:
https://github.com/joshkan/fio/commit/a8f40d4f7013ccd38fc51275d8b00fb4ce2404f5

Patches can be grouped into two parts -
First part (patch 1 and 2) involves extending 'io_uring cmd' infra
(for ioctl updates in task-context), and wire up nvme-passthrough on
char-device (/dev/ngXnY).
The second part (patch 4, 5, 6) introduces fixed-buffer variant of
uring-cmd (IORING_OP_URING_CMD_FIXED), and new NVMe passthrough IOCTLs
that operate on pre-registered buffers.

The buffers are registered with io_uring in the usual fashion. And the
buffer index is passed in command-SQE.
This to be used along with two new fixed-buffer ioctl opcodes:
NVME_IOCTL_IO_CMD_FIXED or NVME_IOCTL_IO64_CMD_FIXED.
For existing passthrough ioctl, nvme-driver maps the buffers in I/O
path for each command.
For the new ioctl, driver supplies buffer-index to io_uring and gets
to reuse pre-mapped buffers.

If introducing new ioctls in nvme (for fixed-buffer) does not sound
great, another option would be to use 'user flags' in the passthrough
command.
I look forward to the thoughts of the community on this.

Jens,

Command-SQE didn't have the field for buf_index. I'm torn among few
options, but repurposed len for now. Please take a look.

I'd appreciate the feedback/comments.


Thanks,

Changes from v4:
https://lore.kernel.org/linux-nvme/20210325170540.59619-1-joshi.k@samsung.com/
1. Moved to v5 branch of Jens, adapted to task-work changes in io_uring
2. Removed support for block-passthrough (over nvme0n1) for now
3. Added support for char-passthrough (over ng0n1)
4. Added fixed-buffer passthrough in io_uring and nvme plumbing

Anuj Gupta (2):
  io_uring: mark iopoll not supported for uring-cmd
  io_uring: add support for uring_cmd with fixed-buffer

Kanchan Joshi (4):
  io_uring: add infra for uring_cmd completion in submitter-task.
  nvme: wire-up support for async-passthru on char-device.
  io_uring: add helper for fixed-buffer uring-cmd
  nvme: enable passthrough with fixed-buffer

 drivers/nvme/host/core.c        |   1 +
 drivers/nvme/host/ioctl.c       | 245 +++++++++++++++++++++++++++++---
 drivers/nvme/host/multipath.c   |   1 +
 drivers/nvme/host/nvme.h        |   5 +
 fs/io_uring.c                   |  69 ++++++++-
 include/linux/io_uring.h        |  15 ++
 include/uapi/linux/io_uring.h   |   6 +-
 include/uapi/linux/nvme_ioctl.h |   2 +
 8 files changed, 319 insertions(+), 25 deletions(-)

-- 
2.25.1