Re: [PATCH rdma-next v3 2/3] RDMA/uverbs: Add DMABUF object type and operations

[Guenter Roeck <linux@roeck-us.net>]

On Sun, Feb 01, 2026 at 04:34:05PM +0200, Edward Srouji wrote:
> From: Yishai Hadas <yishaih@nvidia.com>
> 
> Expose DMABUF functionality to userspace through the uverbs interface,
> enabling InfiniBand/RDMA devices to export PCI based memory regions
> (e.g. device memory) as DMABUF file descriptors. This allows
> zero-copy sharing of RDMA memory with other subsystems that support the
> dma-buf framework.
> 
> A new UVERBS_OBJECT_DMABUF object type and allocation method were
> introduced.
> 
> During allocation, uverbs invokes the driver to supply the
> rdma_user_mmap_entry associated with the given page offset (pgoff).
> 
> Based on the returned rdma_user_mmap_entry, uverbs requests the driver
> to provide the corresponding physical-memory details as well as the
> driver’s PCI provider information.
> 
> Using this information, dma_buf_export() is called; if it succeeds,
> uobj->object is set to the underlying file pointer returned by the
> dma-buf framework.
> 
> The file descriptor number follows the standard uverbs allocation flow,
> but the file pointer comes from the dma-buf subsystem, including its own
> fops and private data.
> 
> When an mmap entry is removed, uverbs iterates over its associated
> DMABUFs, marks them as revoked, and calls dma_buf_move_notify() so that
> their importers are notified.
> 
> The same procedure applies during the disassociate flow; final cleanup
> occurs when the application closes the file.
> 
> Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
> Signed-off-by: Edward Srouji <edwards@nvidia.com>

When trying to build powerpc:ppc64e_defconfig:

ERROR: modpost: "dma_resv_wait_timeout" [drivers/infiniband/core/ib_core.ko] undefined!
ERROR: modpost: "dma_buf_move_notify" [drivers/infiniband/core/ib_core.ko] undefined!
ERROR: modpost: "dma_resv_reset_max_fences" [drivers/infiniband/core/ib_core.ko] undefined!

The code now requires CONFIG_DMA_SHARED_BUFFER which is not enabled for
this platform.

Guenter