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

[javier at javigon.com]

On 26.03.2021 20:59, Niklas Cassel wrote:
>From: Niklas Cassel <niklas.cassel at wdc.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 at wdc.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