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

[Zhiping Zhang <zhipingz@meta.com>]

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 <zhipingz@meta.com>

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 -------- */
 
 /**