Re: [RFC v2 1/2] vfio: add callback to get tph info for dmabuf

Zhiping Zhang
2026-03-31T00:50:01Z

On Fri, Mar 27, 2026 at 7:22 PM fengchengwen <fengchengwen@huawei.com> wrote:
>
> >
> Hi Zhiping,
>
> On 3/25/2026 7:46 AM, Zhiping Zhang wrote:
> > This patch adds a callback to get the tph info on DMA buffer exporters.
> > The tph info includes both the steering tag and the process hint (ph).
> >
> > The steering tag and ph are encoded in the flags field of
> > vfio_device_feature_dma_buf instead of adding new fields to the uapi
> > struct, to preserve ABI compatibility.
> >
> > Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
> > ---
> > drivers/vfio/pci/vfio_pci_dmabuf.c | 26 ++++++++++++++++++++++++--
> > include/linux/dma-buf.h | 30 ++++++++++++++++++++++++++++++
> > include/uapi/linux/vfio.h | 9 +++++++--
> > 3 files changed, 61 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > index 478beafc6ac3..c45cb3884b85 100644
> > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > @@ -17,6 +17,8 @@ struct vfio_pci_dma_buf {
> > struct phys_vec *phys_vec;
> > struct p2pdma_provider *provider;
> > u32 nr_ranges;
> > + u16 steering_tag;
> > + u8 ph;
> > u8 revoked : 1;
> > };
> >
> > @@ -60,6 +62,15 @@ vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> > priv->size, dir);
> > }
> >
> > +static int vfio_pci_dma_buf_get_tph(struct dma_buf *dmabuf, u16 *steering_tag,
> > + u8 *ph)
> > +{
> > + struct vfio_pci_dma_buf *priv = dmabuf->priv;
> > + *steering_tag = priv->steering_tag;
> > + *ph = priv->ph;
>
> If the dmabuf exporter don't provide st&ph, this ops should return error

That is a good call, let me address that in the new revision.

>
> > + return 0;
> > +}
> > +
> > static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> > struct sg_table *sgt,
> > enum dma_data_direction dir)
> > @@ -90,6 +101,7 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
> > .unpin = vfio_pci_dma_buf_unpin,
> > .attach = vfio_pci_dma_buf_attach,
> > .map_dma_buf = vfio_pci_dma_buf_map,
> > + .get_tph = vfio_pci_dma_buf_get_tph,
> > .unmap_dma_buf = vfio_pci_dma_buf_unmap,
> > .release = vfio_pci_dma_buf_release,
> > };
> > @@ -228,7 +240,10 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> > if (copy_from_user(&get_dma_buf, arg, sizeof(get_dma_buf)))
> > return -EFAULT;
> >
> > - if (!get_dma_buf.nr_ranges || get_dma_buf.flags)
> > + if (!get_dma_buf.nr_ranges ||
> > + (get_dma_buf.flags & ~(VFIO_DMABUF_FL_TPH |
> > + VFIO_DMABUF_TPH_PH_MASK |
> > + VFIO_DMABUF_TPH_ST_MASK)))
> > return -EINVAL;
> >
> > /*
> > @@ -285,7 +300,14 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> > ret = PTR_ERR(priv->dmabuf);
> > goto err_dev_put;
> > }
> > -
> > + if (get_dma_buf.flags & VFIO_DMABUF_FL_TPH) {
> > + priv->steering_tag = (get_dma_buf.flags &
> > + VFIO_DMABUF_TPH_ST_MASK) >>
> > + VFIO_DMABUF_TPH_ST_SHIFT;
> > + priv->ph = (get_dma_buf.flags &
> > + VFIO_DMABUF_TPH_PH_MASK) >>
> > + VFIO_DMABUF_TPH_PH_SHIFT;
> > + }
> > /* dma_buf_put() now frees priv */
> > INIT_LIST_HEAD(&priv->dmabufs_elm);
> > down_write(&vdev->memory_lock);
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > index 133b9e637b55..26705c83ad80 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -113,6 +113,36 @@ struct dma_buf_ops {
> > */
> > void (*unpin)(struct dma_buf_attachment *attach);
> >
> > + /**
> > + * @get_tph:
> > + *
> > + * Get the TPH (TLP Processing Hints) for this DMA buffer.
> > + *
> > + * This callback allows DMA buffer exporters to provide TPH including
> > + * both the steering tag and the process hints (ph), which can be used
> > + * to optimize peer-to-peer (P2P) memory access. The TPH info is typically
> > + * used in scenarios where:
> > + * - A PCIe device (e.g., RDMA NIC) needs to access memory on another
> > + * PCIe device (e.g., GPU),
> > + * - The system supports TPH and can use steering tags / ph to optimize
> > + * cache placement and memory access patterns,
> > + * - The memory is exported via DMABUF for cross-device sharing.
> > + *
> > + * @dmabuf: [in] The DMA buffer for which to retrieve TPH
> > + * @steering_tag: [out] Pointer to store the 16-bit TPH steering tag value
> > + * @ph: [out] Pointer to store the 8-bit TPH processing-hint value
> > + *
> > + * Returns:
> > + * * 0 - Success, steering tag stored in @steering_tag
> > + * * -EOPNOTSUPP - TPH steering tags not supported for this buffer
> > + * * -EINVAL - Invalid parameters
> > + *
> > + * This callback is optional. If not implemented, the buffer does not
> > + * support TPH.
>
> It seemed already impl...

Yup, it's supposed to be implemented.

>
> > + *
> > + */
> > + int (*get_tph)(struct dma_buf *dmabuf, u16 *steering_tag, u8 *ph);
> > +
> > /**
> > * @map_dma_buf:
> > *
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > index bb7b89330d35..e2a8962641d2 100644
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -1505,8 +1505,13 @@ struct vfio_region_dma_range {
> > struct vfio_device_feature_dma_buf {
> > __u32 region_index;
> > __u32 open_flags;
> > - __u32 flags;
> > - __u32 nr_ranges;
> > + __u32 flags;
> > +#define VFIO_DMABUF_FL_TPH (1U << 0) /* TPH info is present */
> > +#define VFIO_DMABUF_TPH_PH_SHIFT 1 /* bits 1-2: PH (2-bit) */
> > +#define VFIO_DMABUF_TPH_PH_MASK 0x6U
> > +#define VFIO_DMABUF_TPH_ST_SHIFT 16 /* bits 16-31: steering tag */
> > +#define VFIO_DMABUF_TPH_ST_MASK 0xffff0000U
> > + __u32 nr_ranges;
> > struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
> > };
>
> Another question:
> 1\ PCIE protocol define 8bit and 16bit ST
> 2\ In host-device ST impl, the ACPI will provide 8bit and 16bit ST, the choice of which
> one to use depends on the minimum supported range of the device and the RP.
> 3\ So in this P2P scene, although exporter (e.g. GPU) support 16bit ST, but the consumer
> (e.g. RDMA NIC) only support 8bit this may lead to mis-match
>

Hmm, let me check how we can address this mis-match issue. One option
is to add an
additional parameter and fail the get_tph call when a mis-match is found.

> >
> > --
> > 2.52.0
> >
> >
> >
>