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

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Date:   Thu, 28 Jan 2021 17:34:47 +0530
From:   Kanchan Joshi <joshiiitr@...il.com>
To:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>
Cc:     Kanchan Joshi <joshi.k@...sung.com>,
        Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>, sagi@...mberg.me,
        linux-nvme@...ts.infradead.org, io-uring@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Javier Gonzalez <javier.gonz@...sung.com>,
        Nitesh Shetty <nj.shetty@...sung.com>,
        Selvakumar S <selvakuma.s1@...sung.com>
Subject: Re: [RFC PATCH 0/4] Asynchronous passthrough ioctl

On Wed, Jan 27, 2021 at 9:32 PM Pavel Begunkov <asml.silence@...il.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.
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.

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?


Thanks,