[PATCH v1 00/10] vfio/pci: Allow MMIO regions to be exported through dma-buf

Leon Romanovsky
2025-08-04T13:00:56Z

Changelog:
v1:
* Changed commit messages.
* Reused DMA_ATTR_MMIO attribute.
* Returned support for multiple DMA ranges per-dMABUF.
v0: https://lore.kernel.org/all/cover.1753274085.git.leonro@nvidia.com

---------------------------------------------------------------------------
Based on "[PATCH v1 00/16] dma-mapping: migrate to physical address-based API"
https://lore.kernel.org/all/cover.1754292567.git.leon@kernel.org series.
---------------------------------------------------------------------------

This series extends the VFIO PCI subsystem to support exporting MMIO regions
from PCI device BARs as dma-buf objects, enabling safe sharing of non-struct
page memory with controlled lifetime management. This allows RDMA and other
subsystems to import dma-buf FDs and build them into memory regions for PCI
P2P operations.

The series supports a use case for SPDK where a NVMe device will be owned
by SPDK through VFIO but interacting with a RDMA device. The RDMA device
may directly access the NVMe CMB or directly manipulate the NVMe device's
doorbell using PCI P2P.

However, as a general mechanism, it can support many other scenarios with
VFIO. This dmabuf approach can be usable by iommufd as well for generic
and safe P2P mappings.

In addition to the SPDK use-case mentioned above, the capability added
in this patch series can also be useful when a buffer (located in device
memory such as VRAM) needs to be shared between any two dGPU devices or
instances (assuming one of them is bound to VFIO PCI) as long as they
are P2P DMA compatible.

The implementation provides a revocable attachment mechanism using dma-buf
move operations. MMIO regions are normally pinned as BARs don't change
physical addresses, but access is revoked when the VFIO device is closed
or a PCI reset is issued. This ensures kernel self-defense against
potentially hostile userspace.

The series includes significant refactoring of the PCI P2PDMA subsystem
to separate core P2P functionality from memory allocation features,
making it more modular and suitable for VFIO use cases that don't need
struct page support.

-----------------------------------------------------------------------
The series is based originally on
https://lore.kernel.org/all/20250307052248.405803-1-vivek.kasireddy@intel.com/
but heavily rewritten to be based on DMA physical API.
-----------------------------------------------------------------------
The WIP branch can be found here:
https://git.kernel.org/pub/scm/linux/kernel/git/leon/linux-rdma.git/log/?h=dmabuf-vfio-v1

Thanks

Leon Romanovsky (8):
PCI/P2PDMA: Remove redundant bus_offset from map state
PCI/P2PDMA: Separate the mmap() support from the core logic
PCI/P2PDMA: Simplify bus address mapping API
PCI/P2PDMA: Refactor to separate core P2P functionality from memory
allocation
PCI/P2PDMA: Export pci_p2pdma_map_type() function
types: move phys_vec definition to common header
vfio/pci: Enable peer-to-peer DMA transactions by default
vfio/pci: Add dma-buf export support for MMIO regions

Vivek Kasireddy (2):
vfio: Export vfio device get and put registration helpers
vfio/pci: Share the core device pointer while invoking feature
functions

block/blk-mq-dma.c | 7 +-
drivers/iommu/dma-iommu.c | 4 +-
drivers/pci/p2pdma.c | 154 ++++++++----
drivers/vfio/pci/Kconfig | 20 ++
drivers/vfio/pci/Makefile | 2 +
drivers/vfio/pci/vfio_pci_config.c | 22 +-
drivers/vfio/pci/vfio_pci_core.c | 59 +++--
drivers/vfio/pci/vfio_pci_dmabuf.c | 390 +++++++++++++++++++++++++++++
drivers/vfio/pci/vfio_pci_priv.h | 23 ++
drivers/vfio/vfio_main.c | 2 +
include/linux/dma-buf.h | 1 +
include/linux/pci-p2pdma.h | 114 +++++----
include/linux/types.h | 5 +
include/linux/vfio.h | 2 +
include/linux/vfio_pci_core.h | 4 +
include/uapi/linux/vfio.h | 25 ++
kernel/dma/direct.c | 4 +-
mm/hmm.c | 2 +-
18 files changed, 715 insertions(+), 125 deletions(-)
create mode 100644 drivers/vfio/pci/vfio_pci_dmabuf.c

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2.50.1