[PATCH v4 0/3] vfio/dma-buf: add TPH support for peer-to-peer access

[PATCH v4 0/3] vfio/dma-buf: add TPH support for peer-to-peer access

[Zhiping Zhang]

This series adds TLP Processing Hints (TPH) support to the VFIO dma-buf
export path, allowing importing drivers (e.g. mlx5) to use the
exporter's steering tag when performing peer-to-peer DMA into a
VFIO-owned device.

Patch 1 adds the dma-buf get_tph callback and the new vfio uAPI.
Patch 2 exposes the enabled TPH requester type through a small PCI/TPH
helper so callers don't reach into pci_dev internals.
Patch 3 wires up the mlx5 RDMA driver as a consumer.

Changes since v3:
  - vfio: TPH SET is now write-once per dma-buf; a second
    VFIO_DEVICE_FEATURE_DMA_BUF_TPH on the same dma-buf returns -EBUSY.
  - vfio: annotate priv->vdev with READ_ONCE/WRITE_ONCE across the
    cleanup, release, and feature-set paths to make the existing
    cleanup/release coordination explicit.
  - vfio uAPI: shrink steering_tag from __u16 to __u8 to match the
    8-bit ST width, and drop the trailing reserved[3] in
    struct vfio_device_feature_dma_buf_tph (struct is now 12 bytes).
  - Split the new PCI/TPH helper into its own patch (now patch 2) and
    rename it from pcie_tph_get_st_width() to
    pcie_tph_enabled_req_type(), exposing the enabled requester mode
    rather than just the ST width.
  - mlx5: release the allocated ST index before the MR is pushed back
    to the FRMR pool, so a reused MR cannot reference a freed firmware
    ST entry.
  - mlx5: free mlx5_st_idx_data when its refcount reaches zero, fixing
    a leak that accumulated across repeated alloc/dealloc cycles.
  - Initialize ret in mlx5_st_create() and mlx5_st_alloc_index_by_tag()
    to silence -Wsometimes-uninitialized warnings reported by the
    kernel test robot under clang.

Previous link:
v3: https://lore.kernel.org/linux-pci/20260512184755.4137227-1-zhipingz@meta.com/
v2: https://lore.kernel.org/linux-pci/20260430200704.352228-1-zhipingz@meta.com/

Zhiping Zhang (3):
  vfio: add dma-buf get_tph callback and DMA_BUF_TPH feature
  PCI/TPH: expose the enabled TPH requester type
  RDMA/mlx5: get tph for p2p access when registering dma-buf mr

 drivers/infiniband/hw/mlx5/mlx5_ib.h          |   6 +
 drivers/infiniband/hw/mlx5/mr.c               |  86 ++++++++++-
 .../net/ethernet/mellanox/mlx5/core/lib/st.c  |  28 ++--
 drivers/pci/tph.c                             |  12 ++
 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/linux/mlx5/driver.h                   |   7 +
 include/linux/pci-tph.h                       |   2 +
 include/uapi/linux/vfio.h                     |  35 +++++
 11 files changed, 327 insertions(+), 19 deletions(-)

-- 
2.53.0-Meta

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

[Zhiping Zhang]

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.

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.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 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.
+	 */
+	u32 tph_flags;
+	u16 steering_tag_ext;
+	u8 steering_tag;
+	u8 ph;
+	bool revoked;
 };
 
 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);
@@ -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;
+};
+
 /* -------- API for Type1 VFIO IOMMU -------- */
 
 /**
-- 
2.53.0-Meta

[PATCH v4 2/3] PCI/TPH: expose the enabled TPH requester type

[Zhiping Zhang]

Add pcie_tph_enabled_req_type() so drivers can query the enabled TPH
requester mode without reaching into pci_dev internals. A
!CONFIG_PCIE_TPH stub returns PCI_TPH_REQ_DISABLE so callers need no
ifdef.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/pci/tph.c       | 12 ++++++++++++
 include/linux/pci-tph.h |  2 ++
 2 files changed, 14 insertions(+)

diff --git a/drivers/pci/tph.c b/drivers/pci/tph.c
index 91145e8d9d95..6c4492075ae9 100644
--- a/drivers/pci/tph.c
+++ b/drivers/pci/tph.c
@@ -174,6 +174,18 @@ u32 pcie_tph_get_st_table_loc(struct pci_dev *pdev)
 }
 EXPORT_SYMBOL(pcie_tph_get_st_table_loc);
 
+/**
+ * pcie_tph_enabled_req_type - Return the device's enabled TPH requester type
+ * @pdev: PCI device to query
+ *
+ * Return: PCI_TPH_REQ_DISABLE, PCI_TPH_REQ_TPH_ONLY or PCI_TPH_REQ_EXT_TPH.
+ */
+u8 pcie_tph_enabled_req_type(struct pci_dev *pdev)
+{
+	return pdev->tph_req_type;
+}
+EXPORT_SYMBOL(pcie_tph_enabled_req_type);
+
 /*
  * Return the size of ST table. If ST table is not in TPH Requester Extended
  * Capability space, return 0. Otherwise return the ST Table Size + 1.
diff --git a/include/linux/pci-tph.h b/include/linux/pci-tph.h
index be68cd17f2f8..fe572737b409 100644
--- a/include/linux/pci-tph.h
+++ b/include/linux/pci-tph.h
@@ -30,6 +30,7 @@ void pcie_disable_tph(struct pci_dev *pdev);
 int pcie_enable_tph(struct pci_dev *pdev, int mode);
 u16 pcie_tph_get_st_table_size(struct pci_dev *pdev);
 u32 pcie_tph_get_st_table_loc(struct pci_dev *pdev);
+u8 pcie_tph_enabled_req_type(struct pci_dev *pdev);
 #else
 static inline int pcie_tph_set_st_entry(struct pci_dev *pdev,
 					unsigned int index, u16 tag)
@@ -41,6 +42,7 @@ static inline int pcie_tph_get_cpu_st(struct pci_dev *dev,
 static inline void pcie_disable_tph(struct pci_dev *pdev) { }
 static inline int pcie_enable_tph(struct pci_dev *pdev, int mode)
 { return -EINVAL; }
+static inline u8 pcie_tph_enabled_req_type(struct pci_dev *pdev) { return 0; }
 #endif
 
 #endif /* LINUX_PCI_TPH_H */
-- 
2.53.0-Meta

[PATCH v4 3/3] RDMA/mlx5: get tph for p2p access when registering dma-buf mr

[Zhiping Zhang]

Query dma-buf TPH metadata when registering a dma-buf MR for
peer-to-peer access and translate the returned steering tag into an
mlx5 ST index. The DMAH path keeps priority; dma-buf metadata is the
fallback when no DMAH is supplied.

Track per-MR ownership of the allocated ST index and release it on MR
setup failure, destroy, and before re-entering the FRMR pool. Free
mlx5_st_idx_data when its refcount reaches zero to fix a pre-existing
leak in mlx5_st_dealloc_index().

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/infiniband/hw/mlx5/mlx5_ib.h          |  6 ++
 drivers/infiniband/hw/mlx5/mr.c               | 86 ++++++++++++++++++-
 .../net/ethernet/mellanox/mlx5/core/lib/st.c  | 28 ++++--
 include/linux/mlx5/driver.h                   |  7 ++
 4 files changed, 115 insertions(+), 12 deletions(-)

diff --git a/drivers/infiniband/hw/mlx5/mlx5_ib.h b/drivers/infiniband/hw/mlx5/mlx5_ib.h
index e156dc4d7529..4ab867392267 100644
--- a/drivers/infiniband/hw/mlx5/mlx5_ib.h
+++ b/drivers/infiniband/hw/mlx5/mlx5_ib.h
@@ -721,6 +721,12 @@ struct mlx5_ib_mr {
 			u8 revoked :1;
 			/* Indicates previous dmabuf page fault occurred */
 			u8 dmabuf_faulted:1;
+			/* Set when the MR owns dmabuf_st_index and must
+			 * release it via mlx5_st_dealloc_index() once the
+			 * firmware mkey is no longer referencing it.
+			 */
+			u8 dmabuf_st_owned:1;
+			u16 dmabuf_st_index;
 			struct mlx5_ib_mkey null_mmkey;
 		};
 	};
diff --git a/drivers/infiniband/hw/mlx5/mr.c b/drivers/infiniband/hw/mlx5/mr.c
index 3b6da45061a5..8059b5e4da97 100644
--- a/drivers/infiniband/hw/mlx5/mr.c
+++ b/drivers/infiniband/hw/mlx5/mr.c
@@ -38,6 +38,7 @@
 #include <linux/delay.h>
 #include <linux/dma-buf.h>
 #include <linux/dma-resv.h>
+#include <linux/pci-tph.h>
 #include <rdma/frmr_pools.h>
 #include <rdma/ib_umem_odp.h>
 #include "dm.h"
@@ -46,6 +47,8 @@
 #include "data_direct.h"
 #include "dmah.h"

+MODULE_IMPORT_NS("DMA_BUF");
+
 static int mkey_max_umr_order(struct mlx5_ib_dev *dev)
 {
 	if (MLX5_CAP_GEN(dev->mdev, umr_extended_translation_offset))
@@ -899,6 +902,63 @@ static struct dma_buf_attach_ops mlx5_ib_dmabuf_attach_ops = {
 	.invalidate_mappings = mlx5_ib_dmabuf_invalidate_cb,
 };

+/*
+ * Query TPH metadata from @dmabuf and translate the raw steering tag into
+ * an mlx5 ST index. On success, returns 0 and the caller becomes the
+ * owner of *@st_index (must be released with mlx5_st_dealloc_index()
+ * once the firmware mkey no longer references it). On any failure
+ * *@st_index and *@ph are left as the no-TPH defaults set by the caller.
+ *
+ * @dmabuf must already be referenced by the caller (e.g. via the umem's
+ * attachment) so we don't re-resolve the user's fd here and avoid a
+ * dup2() TOCTOU between umem creation and TPH lookup.
+ */
+static void get_tph_mr_dmabuf(struct mlx5_ib_dev *dev, struct dma_buf *dmabuf,
+			      u16 *st_index, u8 *ph)
+{
+	u8 req_type;
+	u16 steering_tag;
+	u8 st_width;
+	int ret;
+
+	if (!dmabuf->ops->get_tph)
+		return;
+
+	req_type = pcie_tph_enabled_req_type(dev->mdev->pdev);
+	switch (req_type) {
+	case PCI_TPH_REQ_TPH_ONLY:
+		st_width = 8;
+		break;
+	case PCI_TPH_REQ_EXT_TPH:
+		st_width = 16;
+		break;
+	default:
+		return;
+	}
+
+	ret = dmabuf->ops->get_tph(dmabuf, &steering_tag, ph, st_width);
+	if (ret) {
+		mlx5_ib_dbg(dev, "get_tph failed (%d)\n", ret);
+		*ph = MLX5_IB_NO_PH;
+		return;
+	}
+
+	ret = mlx5_st_alloc_index_by_tag(dev->mdev, steering_tag, st_index);
+	if (ret) {
+		*ph = MLX5_IB_NO_PH;
+		mlx5_ib_dbg(dev, "st_alloc_index_by_tag failed (%d)\n", ret);
+	}
+}
+
+static void mlx5_ib_mr_put_dmabuf_st(struct mlx5_ib_mr *mr)
+{
+	if (mr->umem && mr->dmabuf_st_owned) {
+		mlx5_st_dealloc_index(mr_to_mdev(mr)->mdev,
+				      mr->dmabuf_st_index);
+		mr->dmabuf_st_owned = 0;
+	}
+}
+
 static struct ib_mr *
 reg_user_mr_dmabuf(struct ib_pd *pd, struct device *dma_device,
 		   u64 offset, u64 length, u64 virt_addr,
@@ -941,16 +1001,26 @@ reg_user_mr_dmabuf(struct ib_pd *pd, struct device *dma_device,
 		ph = dmah->ph;
 		if (dmah->valid_fields & BIT(IB_DMAH_CPU_ID_EXISTS))
 			st_index = mdmah->st_index;
+	} else {
+		get_tph_mr_dmabuf(dev, umem_dmabuf->attach->dmabuf,
+				  &st_index, &ph);
 	}

 	mr = alloc_cacheable_mr(pd, &umem_dmabuf->umem, virt_addr,
 				access_flags, access_mode,
 				st_index, ph);
 	if (IS_ERR(mr)) {
+		if (!dmah && st_index != MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX)
+			mlx5_st_dealloc_index(dev->mdev, st_index);
 		ib_umem_release(&umem_dmabuf->umem);
 		return ERR_CAST(mr);
 	}

+	if (!dmah && st_index != MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX) {
+		mr->dmabuf_st_index = st_index;
+		mr->dmabuf_st_owned = 1;
+	}
+
 	mlx5_ib_dbg(dev, "mkey 0x%x\n", mr->mmkey.key);

 	atomic_add(ib_umem_num_pages(mr->umem), &dev->mdev->priv.reg_pages);
@@ -1377,9 +1447,17 @@ static int mlx5r_handle_mkey_cleanup(struct mlx5_ib_mr *mr)
 	bool is_odp = is_odp_mr(mr);
 	int ret;

-	if (mr->ibmr.frmr.pool && !mlx5_umr_revoke_mr_with_lock(mr) &&
-	    !ib_frmr_pool_push(mr->ibmr.device, &mr->ibmr))
-		return 0;
+	if (mr->ibmr.frmr.pool && !mlx5_umr_revoke_mr_with_lock(mr)) {
+		/*
+		 * The mkey has been revoked: firmware no longer references
+		 * dmabuf_st_index, so release it before this mr can re-enter
+		 * the FRMR cache for reuse by another registration.
+		 */
+		mlx5_ib_mr_put_dmabuf_st(mr);
+
+		if (!ib_frmr_pool_push(mr->ibmr.device, &mr->ibmr))
+			return 0;
+	}

 	if (is_odp)
 		mutex_lock(&to_ib_umem_odp(mr->umem)->umem_mutex);
@@ -1400,6 +1478,8 @@ static int mlx5r_handle_mkey_cleanup(struct mlx5_ib_mr *mr)
 		dma_resv_unlock(
 			to_ib_umem_dmabuf(mr->umem)->attach->dmabuf->resv);
 	}
+	if (!ret)
+		mlx5_ib_mr_put_dmabuf_st(mr);
 	return ret;
 }

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c
index 997be91f0a13..8929c17c88bc 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c
@@ -29,7 +29,7 @@ struct mlx5_st *mlx5_st_create(struct mlx5_core_dev *dev)
 	u8 direct_mode = 0;
 	u16 num_entries;
 	u32 tbl_loc;
-	int ret;
+	int ret = 0;

 	if (!MLX5_CAP_GEN(dev, mkey_pcie_tph))
 		return NULL;
@@ -92,23 +92,18 @@ void mlx5_st_destroy(struct mlx5_core_dev *dev)
 	kfree(st);
 }

-int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
-			unsigned int cpu_uid, u16 *st_index)
+int mlx5_st_alloc_index_by_tag(struct mlx5_core_dev *dev, u16 tag,
+			       u16 *st_index)
 {
 	struct mlx5_st_idx_data *idx_data;
 	struct mlx5_st *st = dev->st;
 	unsigned long index;
 	u32 xa_id;
-	u16 tag;
-	int ret;
+	int ret = 0;

 	if (!st)
 		return -EOPNOTSUPP;

-	ret = pcie_tph_get_cpu_st(dev->pdev, mem_type, cpu_uid, &tag);
-	if (ret)
-		return ret;
-
 	if (st->direct_mode) {
 		*st_index = tag;
 		return 0;
@@ -152,6 +147,20 @@ int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
 	mutex_unlock(&st->lock);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(mlx5_st_alloc_index_by_tag);
+
+int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
+			unsigned int cpu_uid, u16 *st_index)
+{
+	u16 tag;
+	int ret;
+
+	ret = pcie_tph_get_cpu_st(dev->pdev, mem_type, cpu_uid, &tag);
+	if (ret)
+		return ret;
+
+	return mlx5_st_alloc_index_by_tag(dev, tag, st_index);
+}
 EXPORT_SYMBOL_GPL(mlx5_st_alloc_index);

 int mlx5_st_dealloc_index(struct mlx5_core_dev *dev, u16 st_index)
@@ -175,6 +184,7 @@ int mlx5_st_dealloc_index(struct mlx5_core_dev *dev, u16 st_index)

 	if (refcount_dec_and_test(&idx_data->usecount)) {
 		xa_erase(&st->idx_xa, st_index);
+		kfree(idx_data);
 		/* We leave PCI config space as was before, no mkey will refer to it */
 	}

diff --git a/include/linux/mlx5/driver.h b/include/linux/mlx5/driver.h
index 04b96c5abb57..523a9ab0ae1e 100644
--- a/include/linux/mlx5/driver.h
+++ b/include/linux/mlx5/driver.h
@@ -1166,10 +1166,17 @@ int mlx5_dm_sw_icm_dealloc(struct mlx5_core_dev *dev, enum mlx5_sw_icm_type type
 			   u64 length, u16 uid, phys_addr_t addr, u32 obj_id);

 #ifdef CONFIG_PCIE_TPH
+int mlx5_st_alloc_index_by_tag(struct mlx5_core_dev *dev, u16 tag,
+			       u16 *st_index);
 int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
 			unsigned int cpu_uid, u16 *st_index);
 int mlx5_st_dealloc_index(struct mlx5_core_dev *dev, u16 st_index);
 #else
+static inline int mlx5_st_alloc_index_by_tag(struct mlx5_core_dev *dev,
+					     u16 tag, u16 *st_index)
+{
+	return -EOPNOTSUPP;
+}
 static inline int mlx5_st_alloc_index(struct mlx5_core_dev *dev,
 				      enum tph_mem_type mem_type,
 				      unsigned int cpu_uid, u16 *st_index)
--
2.53.0-Meta

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

[Alex Williamson]

On Tue, 19 May 2026 13:13:49 -0700
Zhiping Zhang <zhipingz@meta.com> 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 <zhipingz@meta.com>
> ---
>  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 -------- */
>  
>  /**

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

[Alex Williamson]

On Thu, 21 May 2026 16:04:12 -0600
Alex Williamson <alex@shazbot.org> wrote:

> On Tue, 19 May 2026 13:13:49 -0700
> Zhiping Zhang <zhipingz@meta.com> 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 <zhipingz@meta.com>
> > ---
> >  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.

On second thought, what dependency does anything here have on
memory_lock?  I think we're jumping through hoops to avoid a lock we
don't even need.  If we just want to serialize SET vs get_tph we could
have a mutex on the dma-buf structure, or use RCU if we want to manage
it locklessly and make sure get_tph always sees a fully consistent set
of values.  Thanks,

Alex

> >  };
> >  
> >  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 -------- */
> >  
> >  /**  
> 

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

[Zhiping Zhang]

On Thu, May 21, 2026 at 3:04 PM Alex Williamson <alex@shazbot.org> wrote:
>
> >
> On Tue, 19 May 2026 13:13:49 -0700
> Zhiping Zhang <zhipingz@meta.com> 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.

Agreed, let me split. v5 will have:
    1/2 dma-buf: add optional get_tph() callback
    2/2 vfio/pci: implement get_tph and VFIO_DEVICE_FEATURE_DMA_BUF_TPH
I will also add Sumit Semwal and Christian König, the dma-buf maintainers.

>
> > 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.

Got it, yes the "set TPH before handing the fd to the importer" contract is a
userspace ordering problem. I'll drop write-once. I'll allow SET to
overwrite and
document the ordering requirement in the uAPI comment instead.

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

You're right: the release/acquire scheme is solving a problem that
doesn't exist.
v5 will drop it; see the reply to your follow-up for the replacement.

>
> > +     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.

Will adopt the bitfield layout you suggested in v5, with the lock annotations.

>
> >  };
> >
> >  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.

Got it, this is fallout from the lockless attempt. priv->vdev
transitions are already
well-ordered by memory_lock. I'll drop all the READ_ONCE/WRITE_ONCE on
priv->vdev in v5 and leave the existing accesses as they were.

>
>
> > @@ -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.

I went back and worked through the layout both ways and I'd actually
like to keep ph as
its own field. I think the separate ph field reads better and costs nothing.

> Thanks,
>
> Alex

Thanks,
Zhiping

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

[Zhiping Zhang]

On Thu, May 21, 2026 at 3:24 PM Alex Williamson <alex@shazbot.org> wrote:
>
> >
> On Thu, 21 May 2026 16:04:12 -0600
> Alex Williamson <alex@shazbot.org> wrote:
>
> > On Tue, 19 May 2026 13:13:49 -0700
> > Zhiping Zhang <zhipingz@meta.com> 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 <zhipingz@meta.com>
> > > ---
> > >  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.
>
> On second thought, what dependency does anything here have on
> memory_lock?  I think we're jumping through hoops to avoid a lock we
> don't even need.  If we just want to serialize SET vs get_tph we could
> have a mutex on the dma-buf structure, or use RCU if we want to manage
> it locklessly and make sure get_tph always sees a fully consistent set
> of values.  Thanks,
>
> Alex

Agreed, we don't need memory_lock in this path. For v5 I'll instead add a
struct mutex lock to struct vfio_pci_dma_buf and take it in SET,
get_tph,
and around the priv->vdev = NULL store in cleanup.

Thanks,
Zhiping

Claude review: vfio/dma-buf: add TPH support for peer-to-peer access

[Claude Code Review Bot]

Overall Series Review

Subject: vfio/dma-buf: add TPH support for peer-to-peer access
Author: Zhiping Zhang <zhipingz@meta.com>
Patches: 8
Reviewed: 2026-05-25T22:27:11.474326

---

This is a well-structured v4 series adding TPH (TLP Processing Hints) support to the VFIO dma-buf export path, enabling importing drivers (mlx5) to use steering tags for peer-to-peer DMA. The series is cleanly split: patch 1 adds the dma-buf callback + VFIO uAPI, patch 2 adds a small PCI/TPH accessor, and patch 3 wires up the mlx5 consumer.

The overall design is sound — write-once TPH metadata with release/acquire ordering, proper fallback to DMAH path, and correct ST index lifetime management. The v4 changelog shows good responsiveness to review feedback (write-once semantics, kfree leak fix, ST index release ordering).

One correctness issue: in patch 3's `get_tph_mr_dmabuf`, the 16-bit ST case writes the returned `u16 steering_tag` into `*st_index` but never reports success/failure to the caller via a return value indicating ST ownership. While this is handled by checking `st_index != MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX` downstream, there is a type mismatch in the get_tph callback. See per-patch details below.

---

---
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Claude review: vfio: add dma-buf get_tph callback and DMA_BUF_TPH feature

[Claude Code Review Bot]

Patch Review

**dma-buf callback design (include/linux/dma-buf.h)**

The `get_tph` callback API is clean. The documentation clearly specifies that 8-bit and 16-bit ST are distinct namespaces. One observation:

```c
int (*get_tph)(struct dma_buf *dmabuf, u16 *steering_tag, u8 *ph,
	       u8 st_width);
```

The `steering_tag` output is `u16 *` but for `st_width == 8` the caller only expects an 8-bit value. This is fine since the VFIO implementation stores `u8 steering_tag` and will only write the low byte, but it means the implementation must be careful. Looking at the actual implementation:

```c
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;
```

This correctly returns the right-width value via the `u16` pointer. Good.

**Write-once semantics and memory ordering (vfio_pci_dmabuf.c)**

The release/acquire pair is correctly used:

```c
/* Writer: */
priv->steering_tag = set_tph.steering_tag;
priv->steering_tag_ext = set_tph.steering_tag_ext;
priv->ph = set_tph.ph;
smp_store_release(&priv->tph_flags, set_tph.flags);

/* Reader: */
flags = smp_load_acquire(&priv->tph_flags);
```

The write-once enforcement via `READ_ONCE(priv->tph_flags)` check returning `-EBUSY` under `memory_lock` is correct.

**READ_ONCE/WRITE_ONCE on priv->vdev**

The conversion of plain accesses to `READ_ONCE`/`WRITE_ONCE` for `priv->vdev` is a reasonable annotation improvement, but there's a subtlety: in `vfio_pci_dma_buf_release`, the code does:

```c
struct vfio_pci_core_device *vdev = READ_ONCE(priv->vdev);
if (vdev) {
	down_write(&vdev->memory_lock);
```

The original code didn't use `READ_ONCE` here either, and this path races with `vfio_pci_dma_buf_cleanup` which does `WRITE_ONCE(priv->vdev, NULL)` under `dma_resv_lock`. The `READ_ONCE` prevents compiler tearing/re-reading but doesn't add synchronization. The existing `kref` protects against double-cleanup, so this is still correct — just worth noting that `READ_ONCE`/`WRITE_ONCE` here are documentation of the existing design rather than a fix.

**uAPI struct (include/uapi/linux/vfio.h)**

```c
struct vfio_device_feature_dma_buf_tph {
	__s32	dmabuf_fd;
	__u32	flags;
	__u8	steering_tag;
	__u8	ph;
	__u16	steering_tag_ext;
};
```

This is 12 bytes total with natural alignment (no holes). Feature number 13 is correctly the next available. The struct has no padding or reserved fields — that's fine for a write-only SET-only feature, but if GET support were ever desired, there'd be no room for extension. Given the write-once semantics, this seems intentional.

**Minor**: The `revoked` field changed from `u8 revoked : 1` to `bool revoked`. This is a clean-up unrelated to TPH — consider splitting it out or at least mentioning it in the commit message.

---
Generated by Claude Code Patch Reviewer

Claude review: PCI/TPH: expose the enabled TPH requester type

[Claude Code Review Bot]

Patch Review

This is a straightforward accessor:

```c
u8 pcie_tph_enabled_req_type(struct pci_dev *pdev)
{
	return pdev->tph_req_type;
}
EXPORT_SYMBOL(pcie_tph_enabled_req_type);
```

The `!CONFIG_PCIE_TPH` stub returns 0, which matches `PCI_TPH_REQ_DISABLE` (defined as `0x0` in `pci_regs.h`):

```c
static inline u8 pcie_tph_enabled_req_type(struct pci_dev *pdev) { return 0; }
```

Clean and correct. The `EXPORT_SYMBOL` (not `_GPL`) matches the existing `pcie_tph_get_st_table_loc` export convention.

**Nit**: The kdoc says "Return: PCI_TPH_REQ_DISABLE, PCI_TPH_REQ_TPH_ONLY or PCI_TPH_REQ_EXT_TPH" — note that `PCI_TPH_REQ_EXT_TPH` is `0x3` (not `0x2`), so value `0x2` is a gap. The function can technically return any value stored in `tph_req_type`, but the current `pcie_enable_tph` only sets these three. Fine as-is.

---
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Claude review: RDMA/mlx5: get tph for p2p access when registering dma-buf mr

[Claude Code Review Bot]

Patch Review

**kfree leak fix (st.c)**

```c
if (refcount_dec_and_test(&idx_data->usecount)) {
	xa_erase(&st->idx_xa, st_index);
+	kfree(idx_data);
	/* We leave PCI config space as was before ... */
}
```

Confirmed — the current tree at `st.c:176-179` has `xa_erase` without `kfree`, leaking `idx_data`. This is a real pre-existing bug fix. Should arguably be a separate patch with its own Fixes: tag since it affects existing users of `mlx5_st_dealloc_index`, not just the new dma-buf path.

**Refactoring mlx5_st_alloc_index (st.c)**

The split into `mlx5_st_alloc_index_by_tag` + a thin wrapper `mlx5_st_alloc_index` is clean. The `int ret = 0` initialization silences the clang warning for the `direct_mode` early-return path where `ret` was previously uninitialized. The export is `_GPL`, matching existing conventions.

**get_tph_mr_dmabuf (mr.c)**

```c
req_type = pcie_tph_enabled_req_type(dev->mdev->pdev);
switch (req_type) {
case PCI_TPH_REQ_TPH_ONLY:
	st_width = 8;
	break;
case PCI_TPH_REQ_EXT_TPH:
	st_width = 16;
	break;
default:
	return;
}
```

This correctly maps the enabled requester type to the ST width for the callback.

**Concern**: After `get_tph` fails, the code does:

```c
if (ret) {
	mlx5_ib_dbg(dev, "get_tph failed (%d)\n", ret);
	*ph = MLX5_IB_NO_PH;
	return;
}
```

This resets `*ph` to `MLX5_IB_NO_PH` but `*st_index` remains `MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX` from the caller's initialization, so the caller won't attempt deallocation. Correct.

**ST index lifecycle (mr.c)**

The `dmabuf_st_owned` flag and `mlx5_ib_mr_put_dmabuf_st` cleanup function correctly handle ownership:

1. On `alloc_cacheable_mr` failure, immediate dealloc before returning error.
2. On success, ownership transferred to MR via `dmabuf_st_owned = 1`.
3. In `mlx5r_handle_mkey_cleanup`, the ST is released after the mkey is revoked but before the MR re-enters the FRMR pool — this ordering is critical and correct.
4. In the non-FRMR cleanup path, `mlx5_ib_mr_put_dmabuf_st` is called after successful mkey cleanup.

```c
if (mr->ibmr.frmr.pool && !mlx5_umr_revoke_mr_with_lock(mr)) {
	mlx5_ib_mr_put_dmabuf_st(mr);
	if (!ib_frmr_pool_push(mr->ibmr.device, &mr->ibmr))
		return 0;
}
```

This ensures the ST index is freed before an MR gets recycled via the FRMR pool, preventing a reused MR from referencing a stale firmware ST entry. Good fix.

**MODULE_IMPORT_NS("DMA_BUF")**

```c
MODULE_IMPORT_NS("DMA_BUF");
```

Required for using dma_buf symbols. Correct.

**Bitfield placement (mlx5_ib.h)**

```c
u8 revoked :1;
u8 dmabuf_faulted:1;
+u8 dmabuf_st_owned:1;
+u16 dmabuf_st_index;
```

Adding these fields inside the anonymous struct within the union is fine, but the `u16 dmabuf_st_index` after three 1-bit `u8` bitfields may introduce padding depending on compiler/ABI. This should be checked — the anonymous struct contains other fields too (`null_mmkey`), so it likely has room, but it's worth verifying that the union size doesn't unexpectedly grow. Not a blocking issue.

**Summary of concerns:**

1. **(Minor)** The `kfree(idx_data)` leak fix in `st.c` is a pre-existing bug — consider splitting it into its own patch with a `Fixes:` tag so it can be backported independently.
2. **(Minor)** The `revoked` type change from bitfield to `bool` in patch 1 is unrelated to TPH and should be mentioned in the commit message.
3. **(Observation)** No `PROBE`/`GET` support for `VFIO_DEVICE_FEATURE_DMA_BUF_TPH` — userspace can't discover whether the feature is available. The `vfio_check_feature` call only allows `SET`. This may be intentional (probe via attempting SET, or probe via the existing DMA_BUF feature), but is worth documenting in the uAPI comment.

---
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