On Thu, 21 May 2026 16:04:12 -0600 Alex Williamson <[email protected]> wrote:
> On Tue, 19 May 2026 13:13:49 -0700 > Zhiping Zhang <[email protected]> wrote: > > > Add a dma-buf get_tph callback for exporters to return TPH > > (TLP Processing Hints) metadata, and add VFIO_DEVICE_FEATURE_DMA_BUF_TPH > > so userspace can attach that metadata to a VFIO-exported dma-buf. > > This should be two patches, the first extending the dma-buf framework > for the get_tph callback for explicit approval from dma-buf maintainers > (who are not even copied here). The second the vfio-pci implementation > of get_tph. > > > 8-bit ST and 16-bit Extended ST are distinct PCIe TPH namespaces; the > > uAPI carries both with explicit validity flags so importers get the > > value matching their requested width. SET is write-once per dma-buf; > > the existing VFIO_DEVICE_FEATURE_DMA_BUF uAPI is unchanged. > > I didn't see what motivated this write-once change, I thought we > understood that it was a userspace problem that the tph values need to > be set before providing the dma-buf fd to the importer and that races > relative to that are a userspace ordering problem. Write-once seems > unnecessarily restrictive and there's no justification provided here. > > > Signed-off-by: Zhiping Zhang <[email protected]> > > --- > > 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 -------- */ > > > > /** >
