[PATCH v2 RFC] nvme: improve performance for virtual NVMe devices

[Christoph Hellwig]

I've not spent much time at it due to a busy conference week, but there
are two main comments on why I don't think this is suitable as is
despite really liking the general idea.

 - I'd really like this to be proposed as an official extension to the
   NVMe technical workgroup.  I'm also happy to help with making that
   happen.  I really don't like to merge it until we have some basic
   agreement on it, although once the basic agreement is there it
   shouldn't be too hard to also support the older google specific
   version.  And this is no new feedback, a couple of people including
   me said that a long time ago, and we've seen zero action on it.
 - the code is a mess in this version.  I really don't see the need for
   all the ifdefs, but if you really want to keep them they should move
   out of the main code path and just stub out helpers that would
   otherwise do work.