Subject: Re: [PATCH v2 1/2] vfio: add dma-buf get_tph callback and DMA_BUF_TPH feature

From: Alex Williamson <[email-masked]>
Date: Mon, 4 May 2026 15:44:59 -0600

On Thu, 30 Apr 2026 13:06:56 -0700
Zhiping Zhang <[email-masked]> wrote:

> Add a dma-buf callback that returns raw TPH metadata from the exporter
> so peer devices can reuse the steering tag and processing hint
> associated with a VFIO-exported buffer.
> 
> Add a new VFIO_DEVICE_FEATURE_DMA_BUF_TPH ioctl that takes the fd from
> VFIO_DEVICE_FEATURE_DMA_BUF along with a steering tag and processing
> hint, validates the fd is a vfio-exported dma-buf belonging to this
> device, and stores the TPH values under memory_lock. This keeps the
> existing VFIO_DEVICE_FEATURE_DMA_BUF uAPI completely unchanged.
> 
> The user sequences setting TPH on the dma-buf before the importer
> consumes it.
> 
> Add an st_width parameter to get_tph() so the exporter can reject
> steering tags that exceed the consumer's supported width (8 vs 16 bit).
> When no TPH metadata was supplied, get_tph() returns -EOPNOTSUPP.
> 
> Signed-off-by: Zhiping Zhang <[email-masked]>

The uAPI is better, but sashiko has some review comments[1] for you.

Please also copy the kvm list for vfio related development.  Thanks,

Alex

[1]https://sashiko.dev/#/patchset/[email-masked]

> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1534,6 +1534,9 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
>  		return vfio_pci_core_feature_token(vdev, flags, arg, argsz);
>  	case VFIO_DEVICE_FEATURE_DMA_BUF:
>  		return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);
> +	case VFIO_DEVICE_FEATURE_DMA_BUF_TPH:
> +		return vfio_pci_core_feature_dma_buf_tph(vdev, flags, arg,
> +							 argsz);
>  	default:
>  		return -ENOTTY;
>  	}
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -19,6 +19,9 @@ struct vfio_pci_dma_buf {
>  	u32 nr_ranges;
>  	struct kref kref;
>  	struct completion comp;
> +	u16 steering_tag;
> +	u8 ph;
> +	u8 tph_present : 1;
>  	u8 revoked : 1;
>  };
>  
> @@ -69,6 +72,22 @@ vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
>  	return ret;
>  }
>  
> +static int vfio_pci_dma_buf_get_tph(struct dma_buf *dmabuf, u16 *steering_tag,
> +				    u8 *ph, u8 st_width)
> +{
> +	struct vfio_pci_dma_buf *priv = dmabuf->priv;
> +
> +	if (!priv->tph_present)
> +		return -EOPNOTSUPP;
> +
> +	if (st_width < 16 && priv->steering_tag > ((1U << st_width) - 1))
> +		return -EINVAL;
> +
> +	*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)
> @@ -101,6 +120,7 @@ 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,
> +	.get_tph = vfio_pci_dma_buf_get_tph,
>  	.map_dma_buf = vfio_pci_dma_buf_map,
>  	.unmap_dma_buf = vfio_pci_dma_buf_unmap,
>  	.release = vfio_pci_dma_buf_release,
> @@ -331,6 +351,55 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  	return ret;
>  }
>  
> +int vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev,
> +				      u32 flags,
> +				      struct vfio_device_feature_dma_buf_tph __user *arg,
> +				      size_t argsz)
> +{
> +	struct vfio_device_feature_dma_buf_tph set_tph;
> +	struct vfio_pci_dma_buf *priv;
> +	struct dma_buf *dmabuf;
> +	int ret;
> +
> +	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET,
> +				 sizeof(set_tph));
> +	if (ret != 1)
> +		return ret;
> +
> +	if (copy_from_user(&set_tph, arg, sizeof(set_tph)))
> +		return -EFAULT;
> +
> +	if (set_tph.reserved)
> +		return -EINVAL;
> +
> +	dmabuf = dma_buf_get(set_tph.dmabuf_fd);
> +	if (IS_ERR(dmabuf))
> +		return PTR_ERR(dmabuf);
> +
> +	if (dmabuf->ops != &vfio_pci_dmabuf_ops) {
> +		ret = -EINVAL;
> +		goto out_put;
> +	}
> +
> +	priv = dmabuf->priv;
> +	down_write(&vdev->memory_lock);
> +	if (priv->vdev != vdev) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +
> +	priv->steering_tag = set_tph.steering_tag;
> +	priv->ph = set_tph.ph;
> +	priv->tph_present = 1;
> +	ret = 0;
> +
> +out_unlock:
> +	up_write(&vdev->memory_lock);
> +out_put:
> +	dma_buf_put(dmabuf);
> +	return ret;
> +}
> +
>  void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
>  {
>  	struct vfio_pci_dma_buf *priv;
> diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
> --- a/drivers/vfio/pci/vfio_pci_priv.h
> +++ b/drivers/vfio/pci/vfio_pci_priv.h
> @@ -118,6 +118,10 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
>  int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  				  struct vfio_device_feature_dma_buf __user *arg,
>  				  size_t argsz);
> +int vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev,
> +				      u32 flags,
> +				      struct vfio_device_feature_dma_buf_tph __user *arg,
> +				      size_t argsz);
>  void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev);
>  void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked);
>  #else
> @@ -128,6 +132,13 @@ vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  {
>  	return -ENOTTY;
>  }
> +static inline int
> +vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev, u32 flags,
> +				  struct vfio_device_feature_dma_buf_tph __user *arg,
> +				  size_t argsz)
> +{
> +	return -ENOTTY;
> +}
>  static inline void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
>  {
>  }
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -113,6 +113,23 @@ struct dma_buf_ops {
>  	 */
>  	void (*unpin)(struct dma_buf_attachment *attach);
>  
> +	/**
> +	 * @get_tph:
> +	 * @dmabuf: DMA buffer for which to retrieve TPH metadata
> +	 * @steering_tag: Returns the raw TPH steering tag
> +	 * @ph: Returns the TPH processing hint
> +	 * @st_width: Consumer's supported steering tag width in bits (8 or 16)
> +	 *
> +	 * Return the TPH (TLP Processing Hints) metadata associated with this
> +	 * DMA buffer. Exporters that do not provide TPH metadata should return
> +	 * -EOPNOTSUPP. If the steering tag exceeds @st_width bits, return
> +	 * -EINVAL.
> +	 *
> +	 * This callback is optional.
> +	 */
> +	int (*get_tph)(struct dma_buf *dmabuf, u16 *steering_tag, u8 *ph,
> +		       u8 st_width);
> +
>  	/**
>  	 * @map_dma_buf:
>  	 *
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1534,6 +1534,28 @@ struct vfio_device_feature_dma_buf {
>   */
>  #define VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2  12
>  
> +/**
> + * Upon VFIO_DEVICE_FEATURE_SET associate TPH (TLP Processing Hints) metadata
> + * with a vfio-exported dma-buf. The dma-buf must have been created by
> + * VFIO_DEVICE_FEATURE_DMA_BUF on this device.
> + *
> + * dmabuf_fd is the file descriptor returned by VFIO_DEVICE_FEATURE_DMA_BUF.
> + * steering_tag and ph are the raw TPH values that importing drivers should use
> + * when accessing the buffer.
> + *
> + * The user must set TPH on the dma-buf before the importer consumes it.
> + *
> + * Return: 0 on success, -errno on failure.
> + */
> +#define VFIO_DEVICE_FEATURE_DMA_BUF_TPH 13
> +
> +struct vfio_device_feature_dma_buf_tph {
> +	__s32	dmabuf_fd;
> +	__u16	steering_tag;
> +	__u8	ph;
> +	__u8	reserved;
> +};
> +
>  /* -------- API for Type1 VFIO IOMMU -------- */
>  
>  /**