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

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

[Keith Busch]

The subject prefix should be lower case "vfio" to match the subsystem
commit style.

On Tue, Feb 10, 2026 at 11:39:54AM -0800, Zhiping Zhang wrote:
> 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);

I don't think you can add fields to a uapi struct like this since it
breaks comptibility. Instead, I think you may be able to carve it out of
the "flags" field since it's currently reserved to be 0, so I guess it's
potentially available to define a new feature.

Re: [RFC 2/2] RMDA MLX5: get tph for p2p access when registering dmabuf mr

[Keith Busch]

On Tue, Feb 10, 2026 at 11:39:55AM -0800, Zhiping Zhang wrote:
> +static void get_tph_mr_dmabuf(struct mlx5_ib_dev *dev, int fd, u16 *st_index,
> +							  u8 *ph)
> +{
> +	int ret;
> +	struct dma_buf *dmabuf;
> +
> +	dmabuf = dma_buf_get(fd);
> +	if (IS_ERR(dmabuf))
> +		return;
> +
> +	if (!dif there's any implication mabuf->ops->get_tph)
> +		goto end_dbuf_put;
> +
> +	ret = dmabuf->ops->get_tph(dmabuf, st_index, ph);

You defined the "get_tph" function to take a pointer to a raw steering
tag value, but you're passing in the steering index to it's table.

But in general, since you're letting the user put whatever they want in
the vfio private area, should there be some validation that it's in the
valid range? I'm also not quite sure how user space comes to know what
steering tag to use, or what harm might happen if the wrong one is used.

Re: [RFC 2/2] RMDA MLX5: get tph for p2p access when registering dmabuf mr

[Jason Gunthorpe]

On Thu, Feb 26, 2026 at 06:21:28PM -0700, Keith Busch wrote:
> On Tue, Feb 10, 2026 at 11:39:55AM -0800, Zhiping Zhang wrote:
> > +static void get_tph_mr_dmabuf(struct mlx5_ib_dev *dev, int fd, u16 *st_index,
> > +							  u8 *ph)
> > +{
> > +	int ret;
> > +	struct dma_buf *dmabuf;
> > +
> > +	dmabuf = dma_buf_get(fd);
> > +	if (IS_ERR(dmabuf))
> > +		return;
> > +
> > +	if (!dif there's any implication mabuf->ops->get_tph)
> > +		goto end_dbuf_put;
> > +
> > +	ret = dmabuf->ops->get_tph(dmabuf, st_index, ph);
> 
> You defined the "get_tph" function to take a pointer to a raw steering
> tag value, but you're passing in the steering index to it's table.

Yeah that's weird, there should be one TPH for a DMABUF, not many.

> But in general, since you're letting the user put whatever they want in
> the vfio private area, should there be some validation that it's in the
> valid range? I'm also not quite sure how user space comes to know what
> steering tag to use, or what harm might happen if the wrong one is used.

If the device is VFIO compatible then it needs to ensure that whatever
it does with its steering tags fit the security model of VFIO. You
can't harm the device - you can't reach outside the VFIO sandbox (eg
into another VF or something) and so on.

Under these conditions the kernel doesn't care what TPH is used, just
let userspace specify the raw bits on the wire.

Jason

[RFC 0/2] Retrieve tph from dmabuf for PCIe P2P memory access

[Zhiping Zhang]

Currently, the steering tag can be used for a CPU on the motherboard; the
ACPI check is in place to query and obtain the supported tph settings. Here
we intend to use the tph info to improve RDMA NIC memory access on a vfio-based
accelerator device via PCIe peer-to-peer. When an applicantion register a
RDMA memory region with DMABUF for the RDMA NIC to access the device memory,
the tph associated with the memory region can be retrieved and used to set the
steering tag / process hint (ph). The tph contains additional instructions
or hints to the GPU or accelerator device for advanced memory operations,
such as, read cache selection.

Note this RFC is for the discussion on the direction and is not intended to be
a complete implementation. Once the direction is agreed on, we will work on the
implementation or a real patch set.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>

[RFC 1/2] Vfio: add callback to get tph info for dmabuf
[RFC 2/2] RMDA MLX5: get tph for p2p access when registering dmabuf

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

[Zhiping Zhang]

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