[RFC 1/2] Vfio: add callback to get tph info for dmabuf

[Zhiping Zhang <zhipingz@meta.com>]

This RFC patch adds a callback to get the tph info on DMA buffer exporters.
The tph info includes both the steering tag and the process hint (ph).

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/vfio/pci/vfio_pci_dmabuf.c | 15 ++++++++++++++-
 include/linux/dma-buf.h            | 30 ++++++++++++++++++++++++++++++
 include/uapi/linux/vfio.h          |  2 ++
 3 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index d4d0f7d08c53..4da1a6cc306f 100644
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -17,6 +17,8 @@ struct vfio_pci_dma_buf {
 	struct dma_buf_phys_vec *phys_vec;
 	struct p2pdma_provider *provider;
 	u32 nr_ranges;
+	u16 steering_tag;
+	u8 ph;
 	u8 revoked : 1;
 };
 
@@ -50,6 +52,15 @@ vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
 				       priv->size, dir);
 }
 
+static int vfio_pci_dma_buf_get_tph(struct dma_buf *dmabuf, u16 *steering_tag,
+				    u8 *ph)
+{
+	struct vfio_pci_dma_buf *priv = dmabuf->priv;
+	*steering_tag = priv->steering_tag;
+	*ph = priv->ph;
+	return 0;
+}
+
 static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
 				   struct sg_table *sgt,
 				   enum dma_data_direction dir)
@@ -78,6 +89,7 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
 static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
 	.attach = vfio_pci_dma_buf_attach,
 	.map_dma_buf = vfio_pci_dma_buf_map,
+	.get_tph = vfio_pci_dma_buf_get_tph,
 	.unmap_dma_buf = vfio_pci_dma_buf_unmap,
 	.release = vfio_pci_dma_buf_release,
 };
@@ -274,7 +286,8 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
 		ret = PTR_ERR(priv->dmabuf);
 		goto err_dev_put;
 	}
-
+	priv->steering_tag = get_dma_buf.steering_tag;
+	priv->ph = get_dma_buf.ph;
 	/* dma_buf_put() now frees priv */
 	INIT_LIST_HEAD(&priv->dmabufs_elm);
 	down_write(&vdev->memory_lock);
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 0bc492090237..466290c02ebf 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -113,6 +113,36 @@ struct dma_buf_ops {
 	 */
 	void (*unpin)(struct dma_buf_attachment *attach);
 
+	/**
+	 * @get_tph:
+	 *
+	 * Get the TPH (TLP Processing Hints) for this DMA buffer.
+	 *
+	 * This callback allows DMA buffer exporters to provide TPH including
+	 * both the steering tag and the process hints (ph), which can be used
+	 * to optimize peer-to-peer (P2P) memory access. The TPH info is typically
+	 * used in scenarios where:
+	 * - A PCIe device (e.g., RDMA NIC) needs to access memory on another
+	 *   PCIe device (e.g., GPU),
+	 * - The system supports TPH and can use steering tags / ph to optimize
+	 *   cache placement and memory access patterns,
+	 * - The memory is exported via DMABUF for cross-device sharing.
+	 *
+	 * @dmabuf: [in] The DMA buffer for which to retrieve TPH
+	 * @steering_tag: [out] Pointer to store the 16-bit TPH steering tag value
+	 * @ph: [out] Pointer to store the 8-bit TPH processing-hint value
+	 *
+	 * Returns:
+	 * * 0 - Success, steering tag stored in @tph
+	 * * -EOPNOTSUPP - TPH steering tags not supported for this buffer
+	 * * -EINVAL - Invalid parameters
+	 *
+	 * This callback is optional. If not implemented, the buffer does not
+	 * support TPH.
+	 *
+	 */
+	int (*get_tph)(struct dma_buf *dmabuf, u16 *steering_tag, u8 *ph);
+
 	/**
 	 * @map_dma_buf:
 	 *
diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index ac2329f24141..bff2f5f7e38d 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -1501,6 +1501,8 @@ struct vfio_region_dma_range {
 struct vfio_device_feature_dma_buf {
 	__u32	region_index;
 	__u32	open_flags;
+	__u16   steering_tag;
+	__u8    ph;
 	__u32   flags;
 	__u32   nr_ranges;
 	struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
-- 
2.47.3