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Date:   Thu, 10 Mar 2022 15:19:45 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Kanchan Joshi <joshiiitr@gmail.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Kanchan Joshi <joshi.k@samsung.com>,
        Jens Axboe <axboe@kernel.dk>, Keith Busch <kbusch@kernel.org>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, sbates@raithlin.com,
        logang@deltatee.com, Pankaj Raghav <pankydev8@gmail.com>,
        Javier =?iso-8859-1?Q?Gonz=E1lez?= <javier@javigon.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Adam Manzanares <a.manzanares@samsung.com>,
        Anuj Gupta <anuj20.g@samsung.com>
Subject: Re: [PATCH 17/17] nvme: enable non-inline passthru commands
Message-ID: <20220310141945.GA890@lst.de>
References: <20220308152105.309618-1-joshi.k@samsung.com> <CGME20220308152729epcas5p17e82d59c68076eb46b5ef658619d65e3@epcas5p1.samsung.com> <20220308152105.309618-18-joshi.k@samsung.com> <20220310083652.GF26614@lst.de> <CA+1E3rLaQstG8LWUyJrbK5Qz+AnNpOnAyoK-7H5foFm67BJeFA@mail.gmail.com>
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On Thu, Mar 10, 2022 at 05:20:13PM +0530, Kanchan Joshi wrote:
> In sync ioctl, we always update this result field by doing put_user on
> completion.
> For async ioctl, since command is inside the the sqe, its lifetime is
> only upto submission. SQE may get reused post submission, leaving no
> way to update the "result" field on completion. Had this field been a
> pointer, we could have saved this on submission and updated on
> completion. But that would require redesigning this structure and
> adding newer ioctl in nvme.

Why would it required adding an ioctl to nvme?  The whole io_uring
async_cmd infrastructure is completely independent from ioctls.

> Coming back, even though sync-ioctl alway updates this result to
> user-space, only a few nvme io commands (e.g. zone-append, copy,
> zone-mgmt-send) can return this additional result (spec-wise).
> Therefore in nvme, when we are dealing with inline-sqe commands from
> io_uring, we never attempt to update the result. And since we don't
> update the result, we limit support to only read/write passthru
> commands. And fail any other command during submission itself (Patch
> 2).

Yikes.  That is outright horrible.  passthrough needs to be command
agnostic and future proof to any newly added nvme command.

> > Overly long line.
> 
> Under 100, but sure, can fold it under 80.

You can only use 100 sparingly if it makes the code more readable.  Which
I know is fuzzy, and in practice never does.  Certainly not in nvme and
block code.