Subject: Re: [PATCH v4 1/3] vfio: add dma-buf get_tph callback and DMA_BUF_TPH feature

From: Alex Williamson <[email-masked]>
Date: Thu, 21 May 2026 16:04:12 -0600

On Tue, 19 May 2026 13:13:49 -0700
Zhiping Zhang <[email-masked]> wrote:

> Add a dma-buf get_tph callback for exporters to return TPH
> (TLP Processing Hints) metadata, and add VFIO_DEVICE_FEATURE_DMA_BUF_TPH
> so userspace can attach that metadata to a VFIO-exported dma-buf.

This should be two patches, the first extending the dma-buf framework
for the get_tph callback for explicit approval from dma-buf maintainers
(who are not even copied here).  The second the vfio-pci implementation
of get_tph.
 
> 8-bit ST and 16-bit Extended ST are distinct PCIe TPH namespaces; the
> uAPI carries both with explicit validity flags so importers get the
> value matching their requested width. SET is write-once per dma-buf;
> the existing VFIO_DEVICE_FEATURE_DMA_BUF uAPI is unchanged.

I didn't see what motivated this write-once change, I thought we
understood that it was a userspace problem that the tph values need to
be set before providing the dma-buf fd to the importer and that races
relative to that are a userspace ordering problem.  Write-once seems
unnecessarily restrictive and there's no justification provided here.
 
> Signed-off-by: Zhiping Zhang <[email-masked]>
> ---
>  drivers/vfio/pci/vfio_pci_core.c   |   3 +
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 134 +++++++++++++++++++++++++++--
>  drivers/vfio/pci/vfio_pci_priv.h   |  12 +++
>  include/linux/dma-buf.h            |  21 +++++
>  include/uapi/linux/vfio.h          |  35 ++++++++
>  5 files changed, 198 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 3f8d093aacf8..94aa6dd95701 100644
> --- 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
> index f87fd32e4a01..be1c65385670 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -19,7 +19,24 @@ struct vfio_pci_dma_buf {
>  	u32 nr_ranges;
>  	struct kref kref;
>  	struct completion comp;
> -	u8 revoked : 1;
> +	/*
> +	 * TPH metadata published by VFIO_DEVICE_FEATURE_DMA_BUF_TPH and
> +	 * consumed by the @get_tph dma-buf callback.
> +	 *
> +	 * @tph_flags is the publish/consume gate: writers populate
> +	 * @steering_tag, @steering_tag_ext and @ph first, then store
> +	 * @tph_flags with smp_store_release(); readers do
> +	 * smp_load_acquire(&tph_flags) before accessing the value fields.
> +	 * @tph_flags == 0 means "TPH not set". Writers publish a non-zero
> +	 * value only once per dma-buf and serialize via vdev->memory_lock;
> +	 * readers stay lockless to avoid AB-BA against the dma_resv_lock held
> +	 * by importers.
> +	 */

Can you outline the ABBA hazard, I'm not seeing it.  You're acquiring
memory_lock in the feature SET and dma_resv_lock doesn't appear to be
held when calling .get_tph().  There's a lot of lockless complication
here balanced on this claim of avoiding a hazard that doesn't appear
present.

> +	u32 tph_flags;
> +	u16 steering_tag_ext;
> +	u8 steering_tag;
> +	u8 ph;
> +	bool revoked;

If we still used memory_lock for tph, these could be:

	u8 tph_st_valid:1; /* memory_lock */
	u8 tph_st_ext_valid:1; /* memory_lock */
	u8 tph_ph:2; /* memory_lock */
	u8 tph_st;
	u16 tph_st_ext;
	u8 revoked:1; /* dma_resv_lock */

The existing change of @revoked from bitfield to bool has no rationale
noted for it in the commit log.

>  };
>  
>  static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> @@ -69,6 +86,36 @@ 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;
> +	u32 flags;
> +
> +	/* Pair with the smp_store_release() in VFIO_DEVICE_FEATURE_DMA_BUF_TPH. */
> +	flags = smp_load_acquire(&priv->tph_flags);
> +	if (!flags)
> +		return -EOPNOTSUPP;
> +
> +	switch (st_width) {
> +	case 8:
> +		if (!(flags & VFIO_DMA_BUF_TPH_ST))
> +			return -EOPNOTSUPP;
> +		*steering_tag = priv->steering_tag;
> +		break;
> +	case 16:
> +		if (!(flags & VFIO_DMA_BUF_TPH_ST_EXT))
> +			return -EOPNOTSUPP;
> +		*steering_tag = priv->steering_tag_ext;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	*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)
> @@ -84,16 +131,17 @@ static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
>  static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
>  {
>  	struct vfio_pci_dma_buf *priv = dmabuf->priv;
> +	struct vfio_pci_core_device *vdev = READ_ONCE(priv->vdev);
>  
>  	/*
>  	 * Either this or vfio_pci_dma_buf_cleanup() will remove from the list.
>  	 * The refcount prevents both.
>  	 */
> -	if (priv->vdev) {
> -		down_write(&priv->vdev->memory_lock);
> +	if (vdev) {
> +		down_write(&vdev->memory_lock);
>  		list_del_init(&priv->dmabufs_elm);
> -		up_write(&priv->vdev->memory_lock);
> -		vfio_device_put_registration(&priv->vdev->vdev);
> +		up_write(&vdev->memory_lock);
> +		vfio_device_put_registration(&vdev->vdev);
>  	}
>  	kfree(priv->phys_vec);
>  	kfree(priv);

This seems unnecessary.  I think this is just because priv->vdev is now
(unnecessarily) set via WRITE_ONCE, right?  These are very well ordered
paths, prior to exposing the dma-buf, while the device is opened, during
release, after release. They don't seem to need the READ/WRITE_ONCE
treatment.  This looks like noise from trying to make it lockless.

> @@ -101,6 +149,7 @@ 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,
> @@ -269,7 +318,7 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  		goto err_free_priv;
>  	}
>  
> -	priv->vdev = vdev;
> +	WRITE_ONCE(priv->vdev, vdev);
>  	priv->nr_ranges = get_dma_buf.nr_ranges;
>  	priv->size = length;
>  	ret = vdev->pci_ops->get_dmabuf_phys(vdev, &priv->provider,
> @@ -331,6 +380,77 @@ 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.flags & ~(VFIO_DMA_BUF_TPH_ST | VFIO_DMA_BUF_TPH_ST_EXT))
> +		return -EINVAL;
> +
> +	if (!set_tph.flags)
> +		return -EINVAL;
> +
> +	/* PCIe TLP Processing Hint is a 2-bit field. */
> +	if (set_tph.ph & ~0x3)
> +		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 (READ_ONCE(priv->vdev) != vdev) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +
> +	/*
> +	 * TPH metadata is write-once per dma-buf so that lockless readers only
> +	 * have to observe a single release-published transition from 0 -> flags.
> +	 */
> +	if (READ_ONCE(priv->tph_flags)) {
> +		ret = -EBUSY;
> +		goto out_unlock;
> +	}
> +
> +	priv->steering_tag = set_tph.steering_tag;
> +	priv->steering_tag_ext = set_tph.steering_tag_ext;
> +	priv->ph = set_tph.ph;
> +	/*
> +	 * Publish the TPH values before the gate flag, so that lockless
> +	 * readers in vfio_pci_dma_buf_get_tph() see fully-initialized
> +	 * fields once they observe a non-zero tph_flags.
> +	 */
> +	smp_store_release(&priv->tph_flags, set_tph.flags);
> +	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;
> @@ -388,7 +508,7 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
>  
>  		dma_resv_lock(priv->dmabuf->resv, NULL);
>  		list_del_init(&priv->dmabufs_elm);
> -		priv->vdev = NULL;
> +		WRITE_ONCE(priv->vdev, NULL);
>  		priv->revoked = true;
>  		dma_buf_invalidate_mappings(priv->dmabuf);
>  		dma_resv_wait_timeout(priv->dmabuf->resv,
> diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
> index fca9d0dfac90..c58f369be4b3 100644
> --- 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 +132,14 @@ 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
> index d1203da56fc5..49eb6ad644a2 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -113,6 +113,27 @@ 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 for @st_width
> +	 * @ph: Returns the TPH processing hint (2-bit value)
> +	 * @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 for the requested steering-tag width. 8-bit ST and 16-bit
> +	 * Extended ST are distinct namespaces in the PCIe TPH ST table and may
> +	 * both be present with different values, so the exporter must select the
> +	 * value that matches @st_width and must not substitute one for the other.
> +	 *
> +	 * Return 0 on success, -EOPNOTSUPP if no metadata is available for the
> +	 * requested width, or -EINVAL if @st_width is not 8 or 16.
> +	 *
> +	 * 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
> index 5de618a3a5ee..a9cb6cbc6ade 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1534,6 +1534,41 @@ 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.
> + *
> + * 8-bit ST (steering_tag) and 16-bit Extended ST (steering_tag_ext) are
> + * distinct namespaces in the PCIe TPH ST table and may both be present with
> + * different values. Userspace should populate the value(s) it has from the
> + * firmware ST table for this device and set the matching VFIO_DMA_BUF_TPH_ST /
> + * VFIO_DMA_BUF_TPH_ST_EXT bit in @flags. An importer requests a specific
> + * width and receives the matching value; if the requested width is not
> + * present, the importer is told TPH is unavailable for this dma-buf.
> + *
> + * ph is the 2-bit TLP Processing Hint and must be in the range [0, 3].
> + *
> + * The user must set TPH on the dma-buf before the importer consumes it.
> + * TPH metadata is write-once per dma-buf; a second SET returns -EBUSY.
> + *
> + * Return: 0 on success, -errno on failure.
> + */
> +#define VFIO_DEVICE_FEATURE_DMA_BUF_TPH 13
> +
> +#define VFIO_DMA_BUF_TPH_ST		(1 << 0)  /* steering_tag valid */
> +#define VFIO_DMA_BUF_TPH_ST_EXT		(1 << 1)  /* steering_tag_ext valid */
> +
> +struct vfio_device_feature_dma_buf_tph {
> +	__s32	dmabuf_fd;
> +	__u32	flags;
> +	__u8	steering_tag;
> +	__u8	ph;
> +	__u16	steering_tag_ext;
> +};

Sure is tempting to make the ph field the first 2-bits of u8 flags.
Thanks,

Alex

> +
>  /* -------- API for Type1 VFIO IOMMU -------- */
>  
>  /**