[PATCH] nvme: support bi-directional commands

[Christoph Hellwig]

On Wed, Dec 07, 2016 at 12:50:50PM -0500, Keith Busch wrote:
> NVMe defines this capability, so why would we want to make it unreachable
> in Linux?

NVMe defines no user of it, so that defintion is entirely theoretical
and it's a giant mess, and your implementation does not actually work
for the general case.

> SG_DXFER_TO_FROM_DEV does bidi commands in one request for
> SCSI using the same rq_map_data method this patch proposes to use.

SG_DXFER_TO_FROM_DEV does not actually work, for SCSI bidi support
you need bsg with the crazy next_rq linked block layer bidi support.