* Re: [PATCH v2 1/2] vfio: add dma-buf get_tph callback and DMA_BUF_TPH feature
2026-05-04 21:44 ` [PATCH v2 1/2] vfio: add dma-buf get_tph callback and DMA_BUF_TPH feature Alex Williamson
@ 2026-05-05 6:54 ` Zhiping Zhang
0 siblings, 0 replies; 2+ messages in thread
From: Zhiping Zhang @ 2026-05-05 6:54 UTC (permalink / raw)
To: Alex Williamson
Cc: Jason Gunthorpe, Leon Romanovsky, Bjorn Helgaas, linux-rdma,
linux-pci, netdev, dri-devel, Keith Busch, Yochai Cohen,
Yishai Hadas, kvm
On Mon, May 4, 2026 at 2:45 PM Alex Williamson <alex@shazbot.org> wrote:
>
> >
> On Thu, 30 Apr 2026 13:06:56 -0700
> Zhiping Zhang <zhipingz@meta.com> wrote:
>
> > Add a dma-buf callback that returns raw TPH metadata from the exporter
> > so peer devices can reuse the steering tag and processing hint
> > associated with a VFIO-exported buffer.
> >
> > Add a new VFIO_DEVICE_FEATURE_DMA_BUF_TPH ioctl that takes the fd from
> > VFIO_DEVICE_FEATURE_DMA_BUF along with a steering tag and processing
> > hint, validates the fd is a vfio-exported dma-buf belonging to this
> > device, and stores the TPH values under memory_lock. This keeps the
> > existing VFIO_DEVICE_FEATURE_DMA_BUF uAPI completely unchanged.
> >
> > The user sequences setting TPH on the dma-buf before the importer
> > consumes it.
> >
> > Add an st_width parameter to get_tph() so the exporter can reject
> > steering tags that exceed the consumer's supported width (8 vs 16 bit).
> > When no TPH metadata was supplied, get_tph() returns -EOPNOTSUPP.
> >
> > Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
>
> The uAPI is better, but sashiko has some review comments[1] for you.
>
> Please also copy the kvm list for vfio related development. Thanks,
>
> Alex
Got it, thanks Alex. let me check sashiko's comments and post a new
revision. i also copied kvm@vger.kernel.org and will include in future
revisions.
Zhiping
>
> [1]https://urldefense.com/v3/__https://sashiko.dev/*/patchset/20260430200704.352228-1-zhipingz@meta.com__;Iw!!Bt8RZUm9aw!7glmqoMRhcdDwOgCAQuuEVqlhFJrh9bAYHXvicXPAO2M-k-NPwE_wFeUjVhe7EXbkXMd6g7eOe13$
>
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -1534,6 +1534,9 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
> > return vfio_pci_core_feature_token(vdev, flags, arg, argsz);
> > case VFIO_DEVICE_FEATURE_DMA_BUF:
> > return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);
> > + case VFIO_DEVICE_FEATURE_DMA_BUF_TPH:
> > + return vfio_pci_core_feature_dma_buf_tph(vdev, flags, arg,
> > + argsz);
> > default:
> > return -ENOTTY;
> > }
> > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > @@ -19,6 +19,9 @@ struct vfio_pci_dma_buf {
> > u32 nr_ranges;
> > struct kref kref;
> > struct completion comp;
> > + u16 steering_tag;
> > + u8 ph;
> > + u8 tph_present : 1;
> > u8 revoked : 1;
> > };
> >
> > @@ -69,6 +72,22 @@ vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> > return ret;
> > }
> >
> > +static int vfio_pci_dma_buf_get_tph(struct dma_buf *dmabuf, u16 *steering_tag,
> > + u8 *ph, u8 st_width)
> > +{
> > + struct vfio_pci_dma_buf *priv = dmabuf->priv;
> > +
> > + if (!priv->tph_present)
> > + return -EOPNOTSUPP;
> > +
> > + if (st_width < 16 && priv->steering_tag > ((1U << st_width) - 1))
> > + return -EINVAL;
> > +
> > + *steering_tag = priv->steering_tag;
> > + *ph = priv->ph;
> > + return 0;
> > +}
> > +
> > static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> > struct sg_table *sgt,
> > enum dma_data_direction dir)
> > @@ -101,6 +120,7 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
> >
> > static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
> > .attach = vfio_pci_dma_buf_attach,
> > + .get_tph = vfio_pci_dma_buf_get_tph,
> > .map_dma_buf = vfio_pci_dma_buf_map,
> > .unmap_dma_buf = vfio_pci_dma_buf_unmap,
> > .release = vfio_pci_dma_buf_release,
> > @@ -331,6 +351,55 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> > return ret;
> > }
> >
> > +int vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev,
> > + u32 flags,
> > + struct vfio_device_feature_dma_buf_tph __user *arg,
> > + size_t argsz)
> > +{
> > + struct vfio_device_feature_dma_buf_tph set_tph;
> > + struct vfio_pci_dma_buf *priv;
> > + struct dma_buf *dmabuf;
> > + int ret;
> > +
> > + ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET,
> > + sizeof(set_tph));
> > + if (ret != 1)
> > + return ret;
> > +
> > + if (copy_from_user(&set_tph, arg, sizeof(set_tph)))
> > + return -EFAULT;
> > +
> > + if (set_tph.reserved)
> > + return -EINVAL;
> > +
> > + dmabuf = dma_buf_get(set_tph.dmabuf_fd);
> > + if (IS_ERR(dmabuf))
> > + return PTR_ERR(dmabuf);
> > +
> > + if (dmabuf->ops != &vfio_pci_dmabuf_ops) {
> > + ret = -EINVAL;
> > + goto out_put;
> > + }
> > +
> > + priv = dmabuf->priv;
> > + down_write(&vdev->memory_lock);
> > + if (priv->vdev != vdev) {
> > + ret = -EINVAL;
> > + goto out_unlock;
> > + }
> > +
> > + priv->steering_tag = set_tph.steering_tag;
> > + priv->ph = set_tph.ph;
> > + priv->tph_present = 1;
> > + ret = 0;
> > +
> > +out_unlock:
> > + up_write(&vdev->memory_lock);
> > +out_put:
> > + dma_buf_put(dmabuf);
> > + return ret;
> > +}
> > +
> > void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
> > {
> > struct vfio_pci_dma_buf *priv;
> > diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
> > --- a/drivers/vfio/pci/vfio_pci_priv.h
> > +++ b/drivers/vfio/pci/vfio_pci_priv.h
> > @@ -118,6 +118,10 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
> > int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> > struct vfio_device_feature_dma_buf __user *arg,
> > size_t argsz);
> > +int vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev,
> > + u32 flags,
> > + struct vfio_device_feature_dma_buf_tph __user *arg,
> > + size_t argsz);
> > void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev);
> > void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked);
> > #else
> > @@ -128,6 +132,13 @@ vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> > {
> > return -ENOTTY;
> > }
> > +static inline int
> > +vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev, u32 flags,
> > + struct vfio_device_feature_dma_buf_tph __user *arg,
> > + size_t argsz)
> > +{
> > + return -ENOTTY;
> > +}
> > static inline void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
> > {
> > }
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -113,6 +113,23 @@ struct dma_buf_ops {
> > */
> > void (*unpin)(struct dma_buf_attachment *attach);
> >
> > + /**
> > + * @get_tph:
> > + * @dmabuf: DMA buffer for which to retrieve TPH metadata
> > + * @steering_tag: Returns the raw TPH steering tag
> > + * @ph: Returns the TPH processing hint
> > + * @st_width: Consumer's supported steering tag width in bits (8 or 16)
> > + *
> > + * Return the TPH (TLP Processing Hints) metadata associated with this
> > + * DMA buffer. Exporters that do not provide TPH metadata should return
> > + * -EOPNOTSUPP. If the steering tag exceeds @st_width bits, return
> > + * -EINVAL.
> > + *
> > + * This callback is optional.
> > + */
> > + int (*get_tph)(struct dma_buf *dmabuf, u16 *steering_tag, u8 *ph,
> > + u8 st_width);
> > +
> > /**
> > * @map_dma_buf:
> > *
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -1534,6 +1534,28 @@ struct vfio_device_feature_dma_buf {
> > */
> > #define VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2 12
> >
> > +/**
> > + * Upon VFIO_DEVICE_FEATURE_SET associate TPH (TLP Processing Hints) metadata
> > + * with a vfio-exported dma-buf. The dma-buf must have been created by
> > + * VFIO_DEVICE_FEATURE_DMA_BUF on this device.
> > + *
> > + * dmabuf_fd is the file descriptor returned by VFIO_DEVICE_FEATURE_DMA_BUF.
> > + * steering_tag and ph are the raw TPH values that importing drivers should use
> > + * when accessing the buffer.
> > + *
> > + * The user must set TPH on the dma-buf before the importer consumes it.
> > + *
> > + * Return: 0 on success, -errno on failure.
> > + */
> > +#define VFIO_DEVICE_FEATURE_DMA_BUF_TPH 13
> > +
> > +struct vfio_device_feature_dma_buf_tph {
> > + __s32 dmabuf_fd;
> > + __u16 steering_tag;
> > + __u8 ph;
> > + __u8 reserved;
> > +};
> > +
> > /* -------- API for Type1 VFIO IOMMU -------- */
> >
> > /**
>
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