Re: [RFC PATCH 0/4] Asynchronous passthrough ioctl

[Pavel Begunkov <asml.silence@gmail.com>]

On 27/01/2021 15:00, Kanchan Joshi wrote:
> 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

see https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-fops

Looks like good time to bring that branch/discussion back

> 
> 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(-)
> 

-- 
Pavel Begunkov