[PATCH v2] nvme: enable FDP support

[Christoph Hellwig]

On Mon, Jun 10, 2024 at 08:52:12AM -0600, Keith Busch wrote:
> I agree the FDP setup is complicated, but none of that is taken on by
> the driver. It just discovers the capabilities and maps an arbitrary
> software "hint" to an arbitrary device "hint". It's up to the
> application to use those optimally; the driver just performs the
> requested mapping.
> 
> Is it because the names of those hints indicate data lifetime? These are
> just arbitrary numbers used by applications to separate placement. If
> they were called HINT_A, HINT_B, HINT_C, would that make this ok?

No, the other problem is that FDP very much has an implicit contract
that the host actually aligns to it resources units, and actually
has a really complicated mangement.  It's not a simple throw a
lifetime hint at the drive.  Note that the implicit is indeed very
implicit - it is a really horrible spec with a lot of assumptions
but nothing actually enforcing it.  If you just use it for dumb
lifetime hints changes are that you actually increase write
amplificiation.