[RFC PATCH 0/5] vfio/pci: Support ZONE_DEVICE-backed P2P Registration

[Pranjal Shrivastava <praan@google.com>]

Introduce ZONE_DEVICE backing for VFIO-exposed PCIe BARs.
Currently, kernel drivers can register their BARs with the P2PDMA
subsystem to enable high-performance, page-backed P2P DMA. However, when
a device is bound to vfio-pci, this capability is missing.

As a framework, VFIO should support the same set of capabilities 
available to kernel drivers, provided appropriate isolation and 
lifecycles are maintained.

This series adds support for VFIO to register BARs as page-backed P2P
providers, enabling userspace drivers to perform zero-copy P2P DMA via
standard POSIX APIs (e.g., O_DIRECT) which require struct page metadata.

Design
======
The proposed design enables VFIO to leverage the kernel's pci-p2pdma 
infrastructure, delegating memory management and slicing to the managed
P2P pool.

1. ZONE_DEVICE Registration
A new feature, VFIO_DEVICE_FEATURE_P2P_REGISTER, is introduced to trigger
pci_p2pdma_add_resource() for a target BAR which creates the ZONE_DEVICE
struct pages and registers the BAR as a standard P2P provider.

2. Mutual Exclusivity Policy
A strict mutual exclusion policy is enforced:
     - Registered BARs: Standard mmap() and DMABUF export are blocked.
     - Exported BARs: Native P2P registration is blocked.

Users utilize the standard sysfs p2pmem/allocate interface for managing
memory slices once a BAR is registered.

3. Lifecycle and Resource Reclamation
A new helper is introduced: pci_p2pdma_remove_resource(), is implemented
to support manual teardown of the P2P pool. VFIO invokes this helper 
during device reset or session termination to ensure the pool is
destroyed and the ZONE_DEVICE pages are unmapped.

This is on github: https://github.com/pran005/linux/tree/vfio-p2p

Alternative Approach: Page-backed DMABUF Export
================================================
An alternative implementation has been explored which integrates with the
ongoing VFIO DMABUF-mmap refactor [1]. In that approach, rather than
registering a BAR as a system-wide P2P provider, VFIO optionally
allocates ZONE_DEVICE pages only for specifically exported DMABUFs via a
new VFIO_DMA_BUF_FLAG_ALLOC_STRUCT_PAGES flag.

A proof-of-concept of this approach is available at:
https://github.com/pran005/linux/tree/vfio-p2p-alt

[1] https://lore.kernel.org/all/20260527102319.100128-1-mattev@meta.com/

Pranjal Shrivastava (5):
  vfio: Add UAPI for ZONE_DEVICE-backed P2P registration
  vfio/pci: Implement ZONE_DEVICE-backed P2P registration
  vfio/pci: Block mmap & dmabuf export for ZONE_DEVICE-registered BARs
  vfio/pci: Block ZONE_DEVICE registration for BARs with active DMABUFs
  PCI/P2PDMA: Introduce a helper to release P2P resources

 drivers/pci/p2pdma.c               | 38 ++++++++++++++++++++
 drivers/vfio/pci/vfio_pci_core.c   | 57 ++++++++++++++++++++++++++++++
 drivers/vfio/pci/vfio_pci_dmabuf.c | 22 ++++++++++++
 drivers/vfio/pci/vfio_pci_priv.h   |  6 ++++
 include/linux/pci-p2pdma.h         |  1 +
 include/linux/vfio_pci_core.h      |  1 +
 include/uapi/linux/vfio.h          | 15 ++++++++
 7 files changed, 140 insertions(+)

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2.54.0.1099.g489fc7bff1-goog