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

[javier@xxxxxxxxxxx]

On 26.03.2021 20:59, Niklas Cassel wrote:
> From: Niklas Cassel <niklas.cassel@xxxxxxx>
>
> 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@xxxxxxx>

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