Re: [PATCH v5 0/4] vfio/dma-buf: add TPH support for peer-to-peer access

["Christian König" <christian.koenig@amd.com>]

On 5/26/26 16:43, Zhiping Zhang wrote:
> 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.

I'm not an expert for TPH, but that sounds very strange to me.

As far as I know the TLP Processing Hints allow devices to give a steering tag to the root complex together with memory accesses to give fine grained control about cache usage. In other words it is an extension to the classic snoop bit.

For P2P that is obviously nonsense because we don't have P2P support for cached accesses.

So what puzzle piece I'm missing?

Regards,
Christian.

> 
> Patch 1 exposes the enabled TPH requester type through a small PCI/TPH
> helper so callers don't reach into pci_dev internals.
> Patch 2 adds the optional dma_buf_ops::get_tph callback to the dma-buf
> framework so importers can fetch TPH metadata from an exporter.
> Patch 3 implements get_tph in vfio-pci and adds the new uAPI
> (VFIO_DEVICE_FEATURE_DMA_BUF_TPH) for userspace to attach the metadata.
> Patch 4 wires up the mlx5 RDMA driver as a consumer.
> 
> Previous link:
> v4: https://lore.kernel.org/linux-pci/20260519201401.1558410-1-zhipingz@meta.com/
> 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 (4):
>   PCI/TPH: expose the enabled TPH requester type
>   dma-buf: add optional get_tph() callback
>   vfio/pci: implement get_tph and DMA_BUF_TPH feature
>   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            | 110 +++++++++++++++++-
>  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                     |  37 ++++++
>  11 files changed, 311 insertions(+), 13 deletions(-)
> 
> --
> 2.53.0-Meta
>