[PATCH] nvme: support bi-directional commands

[Christoph Hellwig]

On Wed, Dec 07, 2016 at 12:39:24PM -0500, Keith Busch wrote:
> The nvme specification defines the opcode's lower 2 bits as the
> transfer direction, which allows for bi-directional commands. While
> there are no standard defined opcodes that use both data directions,
> this is something a vendor unique opcode may use.
> 
> This patch adds support for bi-directional user commands. The block
> layer doesn't natively support a request with both directions, but we
> can treat it like a read and set up rq_map_data to force copying the
> user data to the kernel buffers before the transfer.

The block layer actually supports bidi commands for SCSI OSD devices
using two struct requests.  But it's giant mess, and given that no
NVMe command in the spec requires it I am absoutely 100% against
supporting this in NVMe.  It's just going to create a mess for everyone
involved, even worse so for fabrics.

Just don't do it.