[PATCH v2 0/1] nvdimm: allow exposing RAM as libnvdimm DIMMs

[Mike Rapoport <rppt@kernel.org>]

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

Hi,

It's not uncommon that libnvdimm/dax/ndctl are used with normal volatile
memory for a whole bunch of $reasons.

Probably the most common usecase is to back VMs memory with fsdax/devdax,
but there are others as well when there's a requirement to manage memory
separately from the kernel.

The existing mechanisms to expose normal ram as "persistent", such as
memmap=x!y on x86 or dummy pmem-region device tree nodes on DT systems lack
flexibility to dynamically partition a single region without rebooting the
system and sometimes even updating the system firmware. Also, to create
several DAX devices with different properties it's necessary to repeat
the memmap= command line option or add several pmem-region nodes to the
DT.

I propose a new ramdax driver that will create a DIMM device on
E820_TYPE_PRAM/pmem-region and that will allow partitioning that device
dynamically. The label area is kept in the end of that region and managed
by the driver.

v2 changes:
* Change the way driver is bound to a device, following Dan's
  suggestion. Instead of forcing mutual exclusion of ramdax and
  nr_e820/of-pmem at build time, rely on 'driver_override' attribute to
  allow binding ramdax driver to e820_pmem/pmem-region devices.
* Fix build warning reported by kbuild

v1: https://lore.kernel.org/all/20250826080430.1952982-1-rppt@kernel.org
* fix offset calculations in ramdax_{get,set}_config_data
* use a magic constant instead of a random number as nd_set->cookie*

RFC: https://lore.kernel.org/all/20250612083153.48624-1-rppt@kernel.org

Mike Rapoport (Microsoft) (1):
  nvdimm: allow exposing RAM carveouts as NVDIMM DIMM devices

 drivers/nvdimm/Kconfig  |  17 +++
 drivers/nvdimm/Makefile |   1 +
 drivers/nvdimm/ramdax.c | 272 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 290 insertions(+)
 create mode 100644 drivers/nvdimm/ramdax.c


base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
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2.50.1