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

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Date:   Wed, 27 Jan 2021 15:53:19 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Kanchan Joshi <joshi.k@...sung.com>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me
Cc:     linux-nvme@...ts.infradead.org, io-uring@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        javier.gonz@...sung.com, nj.shetty@...sung.com,
        selvakuma.s1@...sung.com
Subject: Re: [RFC PATCH 0/4] Asynchronous passthrough ioctl

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

-- 
Pavel Begunkov