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From: Kanchan Joshi <[email protected]>
To: Jens Axboe <[email protected]>
Cc: Pavel Begunkov <[email protected]>,
	Kanchan Joshi <[email protected]>,
	Keith Busch <[email protected]>, Christoph Hellwig <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected],
	Javier Gonzalez <[email protected]>,
	Nitesh Shetty <[email protected]>,
	Selvakumar S <[email protected]>
Subject: Re: [RFC PATCH 0/4] Asynchronous passthrough ioctl
Date: Thu, 28 Jan 2021 22:43:26 +0530	[thread overview]
Message-ID: <CA+1E3rKeqaLXBuvpMcjZ37XH9RqJHjPnTFObJj0T-u8K9Otw-w@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>

On Thu, Jan 28, 2021 at 8:08 PM Jens Axboe <[email protected]> wrote:
>
> On 1/28/21 5:04 AM, Kanchan Joshi wrote:
> > On Wed, Jan 27, 2021 at 9:32 PM Pavel Begunkov <[email protected]> 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

  reply	other threads:[~2021-01-28 17:17 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20210127150134epcas5p251fc1de3ff3581dd4c68b3fbe0b9dd91@epcas5p2.samsung.com>
2021-01-27 15:00 ` [RFC PATCH 0/4] Asynchronous passthrough ioctl Kanchan Joshi
     [not found]   ` <CGME20210127150140epcas5p32832cc0c0db953db199eb9dd326f2d4c@epcas5p3.samsung.com>
2021-01-27 15:00     ` [RFC PATCH 1/4] block: introduce async ioctl operation Kanchan Joshi
     [not found]   ` <CGME20210127150144epcas5p29ccb35d7e7170aba7947b5ee16fd2db0@epcas5p2.samsung.com>
2021-01-27 15:00     ` [RFC PATCH 2/4] kernel: export task_work_add Kanchan Joshi
     [not found]   ` <CGME20210127150149epcas5p4fa8edd47712f28ccdd9bac5139fc6e61@epcas5p4.samsung.com>
2021-01-27 15:00     ` [RFC PATCH 3/4] nvme: add async ioctl support Kanchan Joshi
     [not found]   ` <CGME20210127150156epcas5p26cdf368e4ff6bffb132fa1c7f9430653@epcas5p2.samsung.com>
2021-01-27 15:00     ` [RFC PATCH 4/4] io_uring: add async passthrough " Kanchan Joshi
2021-01-27 15:42   ` [RFC PATCH 0/4] Asynchronous passthrough ioctl Pavel Begunkov
2021-01-27 15:53     ` Pavel Begunkov
2021-01-28 12:04       ` Kanchan Joshi
2021-01-28 14:38         ` Jens Axboe
2021-01-28 17:13           ` Kanchan Joshi [this message]
2021-01-28 17:24             ` Jens Axboe
2021-02-22 13:42               ` Kanchan Joshi
2021-02-22 14:33                 ` Jens Axboe
2021-02-23  4:41                   ` Kanchan Joshi
2021-01-28 14:50         ` Jens Axboe
2021-01-28 17:25           ` Kanchan Joshi

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