[RFC PATCH 0/4] Asynchronous passthrough ioctl

[Kanchan Joshi]

This RFC patchset adds asynchronous ioctl capability for NVMe devices.
Purpose of RFC is to get the feedback and optimize the path.

At the uppermost io-uring layer, a new opcode IORING_OP_IOCTL_PT is
presented to user-space applications. Like regular-ioctl, it takes
ioctl opcode and an optional argument (ioctl-specific input/output
parameter). Unlike regular-ioctl, it is made to skip the block-layer
and reach directly to the underlying driver (nvme in the case of this
patchset). This path between io-uring and nvme is via a newly
introduced block-device operation "async_ioctl". This operation
expects io-uring to supply a callback function which can be used to
report completion at later stage.

For a regular ioctl, NVMe driver submits the command to the device and
the submitter (task) is made to wait until completion arrives. For
async-ioctl, completion is decoupled from submission. Submitter goes
back to its business without waiting for nvme-completion. When
nvme-completion arrives, it informs io-uring via the registered
completion-handler. But some ioctls may require updating certain
ioctl-specific fields which can be accessed only in context of the
submitter task. For that reason, NVMe driver uses task-work infra for
that ioctl-specific update. Since task-work is not exported, it cannot
be referenced when nvme is compiled as a module. Therefore, one of the
patch exports task-work API.

Here goes example of usage (pseudo-code).
Actual nvme-cli source, modified to issue all ioctls via this opcode
is present at-
https://github.com/joshkan/nvme-cli/commit/a008a733f24ab5593e7874cfbc69ee04e88068c5

With regular ioctl-
int nvme_submit_passthru(int fd, unsigned long ioctl_cmd,
                         struct nvme_passthru_cmd *cmd)
{
	return ioctl(fd, ioctl_cmd, cmd);
}

With uring passthru ioctl-
int nvme_submit_passthru(int fd, unsigned long ioctl_cmd,
                         struct nvme_passthru_cmd *cmd)
{
	return uring_ioctl(fd, ioctl_cmd, cmd);
}
int uring_ioctl(int fd, unsinged long cmd, u64 arg)
{
	sqe = io_uring_get_sqe(ring);

	/* prepare sqe */
	sqe->fd = fd;
	sqe->opcode = IORING_OP_IOCTL_PT;
	sqe->ioctl_cmd = cmd;
	sqe->ioctl_arg = arg;

	/* submit sqe */
	io_uring_submit(ring);

	/* reap completion and obtain result */
	io_uring_wait_cqe(ring, &cqe);
	printf("ioctl result =%d\n", cqe->res)
}

Kanchan Joshi (4):
  block: introduce async ioctl operation
  kernel: export task_work_add
  nvme: add async ioctl support
  io_uring: add async passthrough ioctl support

 drivers/nvme/host/core.c      | 347 +++++++++++++++++++++++++++-------
 fs/io_uring.c                 |  77 ++++++++
 include/linux/blkdev.h        |  12 ++
 include/uapi/linux/io_uring.h |   7 +-
 kernel/task_work.c            |   2 +-
 5 files changed, 376 insertions(+), 69 deletions(-)

-- 
2.25.1