linux-kernel - Re: [RFC PATCH] nvme: allow NVME_IOCTL_IO_CMD on controller char dev even when multiple ns

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Date:   Tue, 30 Mar 2021 20:30:22 +0200
From:   "javier@...igon.com" <javier@...igon.com>
To:     Niklas Cassel <Niklas.Cassel@....com>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "minwoo.im.dev@...il.com" <minwoo.im.dev@...il.com>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] nvme: allow NVME_IOCTL_IO_CMD on controller char dev
 even when multiple ns

On 26.03.2021 20:59, Niklas Cassel wrote:
>From: Niklas Cassel <niklas.cassel@....com>
>
>Currently when doing NVME_IOCTL_IO_CMD on the controller character device,
>the command is rejected if there is more than one namespace in the
>ctrl->namespaces list.
>
>There is not really any reason for this restriction.
>Instead, check the nsid value specified in the passthru command, and try
>to find the matching namespace in ctrl->namespaces list.
>If found, call nvme_user_cmd() on the namespace.
>If not found, reject the command.
>
>While at it, remove the warning that says that NVME_IOCTL_IO_CMD is
>deprecated on the controller character device.
>There is no comment saying why it is deprecated.
>It might be very unsafe to send a passthru command, but if that is
>the issue with this IOCTL, then we should add a warning about that
>instead.
>
>Signed-off-by: Niklas Cassel <niklas.cassel@....com>

I think the idea is OK, but I have 3 questions:

   1. Wouldn't this break user-space when nsid is not specified?
   2. What is the use case for this? As I understand it, this char device
   is primarily for admin commands sent to the controller. Do you see a
   use case for sending commands to the namespace using the controller
   char device?
   3. Following up on the above, if the use-case is I/O, don't you think
   it is cleaner to use the ongoing per-namespace char device effort? We
   would very much like to get your input there and eventually send a
   series together. When this is merged, we could wire that logic to the
   controller char device if there is an use-case for it.

Javier