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

[Kanchan Joshi <joshiiitr@gmail.com>]

On Thu, Jan 28, 2021 at 8:08 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 1/28/21 5:04 AM, Kanchan Joshi wrote:
> > On Wed, Jan 27, 2021 at 9:32 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
> >>
> >> On 27/01/2021 15:42, Pavel Begunkov wrote:
> >>> 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
> >>
> >> a bit more context:
> >> https://github.com/axboe/liburing/issues/270
> >
> > Thanks, it looked good. It seems key differences (compared to
> > uring-patch that I posted) are -
> > 1. using file-operation instead of block-dev operation.
>
> Right, it's meant to span wider than just block devices.
>
> > 2. repurpose the sqe memory for ioctl-cmd. If an application does
> > ioctl with <=40 bytes of cmd, it does not have to allocate ioctl-cmd.
> > That's nifty. We still need to support passing larger-cmd (e.g.
> > nvme-passthru ioctl takes 72 bytes) but that shouldn't get too
> > difficult I suppose.
>
> It's actually 48 bytes in the as-posted version, and I've bumped it to
> 56 bytes in the latest branch. So not quite enough for everything,
> nothing ever will be, but should work for a lot of cases without
> requiring per-command allocations just for the actual command.

Agreed. But if I got it right, you are open to support both in-the-sqe
command (<= 56 bytes) and out-of-sqe command (> 56 bytes) with this
interface.
Driver processing the ioctl can fetch the cmd from user-space in one
case (as it does now), and skips in another.

> > And for some ioctls, driver may still need to use task-work to update
> > the user-space pointers (embedded in uring/ioctl cmd) during
> > completion.
> >
> > @Jens - will it be fine if I start looking at plumbing nvme-part of
> > this series on top of your work?
>
> Sure, go ahead. Just beware that things are still changing, so you might
> have to adapt it a few times. It's still early days, but I do think
> that's the way forward in providing controlled access to what is
> basically async ioctls.

Sounds good, I will start with the latest branch that you posted. Thanks.

-- 
Kanchan