[PATCH RFC 0/5] nvme: Controller Data Queue (CDQ) support

[Joel Granados <joel.granados@kernel.org>]

This RFC implements Controller Data Queue (CDQ) support in the NVMe
driver, a variation of my original RFC sent last July [2]. It exposes an
ioctl interface for userspace to create, configure, and delete CDQs
backed by DMA-mapped user memory with eventfd notification. In this
version I explore how the CDQ protocol logic might live outside the
kernel; the ioctl serves as a testing tool but is not necessarily the
final interface.

This RFC exists within a broader goal, which is to enable NVMe namespace
migration. The timing feels right as hardware with CDQ capability
exists, NVMe fully specifies the feature and there is growing interest
in Live Migration which by extension includes CDQ.

There is however, no clear consensus on how NVMe Live Migration should
land in the Linux kernel. The 2022 discussion [1] explored a VFIO-based
approach but reached no conclusion, likely because the specification was
not yet mature.

To move CDQ forward, I would like to understand where the LM logic belongs. I
currently see two options (of which I have no particular preference):

1. VFIO: Implement NVMe LM following the VFIO state machine, similar to what
   was proposed in 2022.
2. VM manager interface: Bypass VFIO and implement LM logic in the interface
   between the VM manager (e.g., QEMU) and the NVMe driver.

One aspect that has not received much attention in previous discussions
is namespace migration as prior work focused on migrating state and not
the actual data. Migrating potential terabytes is IMO a distinct use
case worth considering. LSF/MM/BPF is in a week. I hope this series
encourages folks to revisit their positions, give their opinions and set
the stage for face2face discussions.

Best

PS: I'm including the regular NVMe contacts and the folks that seemed to
have strong opinions in [2]. I always find it difficult to decide who to
include in these so let me know if you want to be removed in the future
or if I have missed someone.

[1] https://lore.kernel.org/20221206055816.292304-1-lei.rao@intel.com
[2] https://lore.kernel.org/20250714-jag-cdq-v1-0-01e027d256d5@kernel.org

Signed-off-by: Joel Granados <joel.granados@kernel.org>
---
Joel Granados (5):
      nvme: Add CDQ data structures to nvme spec header
      nvme: Add CDQ data structures to host driver
      nvme: Add NVME_AER_ONE_SHOT callback handler
      nvme: Implement CDQ core functionality
      nvme: Add CDQ ioctl interface

 drivers/nvme/host/core.c        | 312 ++++++++++++++++++++++++++++++++++++++++
 drivers/nvme/host/ioctl.c       |  53 ++++++-
 drivers/nvme/host/nvme.h        |  20 +++
 include/linux/nvme.h            |  50 ++++++-
 include/uapi/linux/nvme_ioctl.h |  29 ++++
 5 files changed, 462 insertions(+), 2 deletions(-)
---
base-commit: 028ef9c96e96197026887c0f092424679298aae8
change-id: 20260424-jag-cdq-lkml-cd9b7c79983d

Best regards,
-- 
Joel Granados <joel.granados@kernel.org>