iommufd: Clarify IOAS_MAP_FILE dma-buf support

IOMMU_IOAS_MAP_FILE is documented as mapping a memfd, but the
implementation first tries to resolve the fd as a dma-buf and has a
special path for supported dma-buf exporters. In particular, VFIO PCI
dma-bufs exported through VFIO_DEVICE_FEATURE_DMA_BUF can be mapped when
they describe a single DMA range.

Update the UAPI comment so userspace understands that certain kinds of
dma-buf are supported in addition to memfd.

Fixes: 44ebaa1744fd ("iommufd: Accept a DMABUF through IOMMU_IOAS_MAP_FILE")
Link: https://patch.msgid.link/r/20260610-tmp-v1-1-b8ccbf557391@fb.com
Signed-off-by: Alex Mastro <amastro@fb.com>
Assisted-by: Codex:gpt-5.5-high
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
index e998dfbd69603149a09b6a2e47223249423eaf1e..0425d452d41ed6f862d09ef2bc7163121bc5c8a6 100644 (file)
--- a/include/uapi/linux/iommufd.h
+++ b/include/uapi/linux/iommufd.h
@@ -224,13 +224,17 @@ struct iommu_ioas_map {
  * @size: sizeof(struct iommu_ioas_map_file)
  * @flags: same as for iommu_ioas_map
  * @ioas_id: same as for iommu_ioas_map
- * @fd: the memfd to map
- * @start: byte offset from start of file to map from
+ * @fd: the memfd or supported dma-buf file to map
+ * @start: byte offset from start of the file to map from
  * @length: same as for iommu_ioas_map
  * @iova: same as for iommu_ioas_map
  *
- * Set an IOVA mapping from a memfd file.  All other arguments and semantics
- * match those of IOMMU_IOAS_MAP.
+ * Set an IOVA mapping from a memfd file. On kernels with dma-buf support,
+ * supported dma-buf files may also be accepted. This is not a generic
+ * dma-buf import path; currently supported dma-bufs include single-range
+ * VFIO PCI dma-bufs exported through VFIO_DEVICE_FEATURE_DMA_BUF, and
+ * other dma-bufs may be rejected. All other arguments and semantics match
+ * those of IOMMU_IOAS_MAP.
  */
 struct iommu_ioas_map_file {
        __u32 size;