[PATCH] iommufd: Clarify IOAS_MAP_FILE dma-buf support

[Alex Mastro <amastro@fb.com>]

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")
Signed-off-by: Alex Mastro <amastro@fb.com>
Assisted-by: Codex:gpt-5.5-high
---
 include/uapi/linux/iommufd.h | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
index e998dfbd6960..0425d452d41e 100644
--- 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;

---
base-commit: 61d525d96ade3367ce65e340b33aa2f1fd5a3602
change-id: 20260610-tmp-b39b94ec1ff4

Best regards,
--  
Alex Mastro <amastro@fb.com>