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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, May 27, 2026 at 08:55:49AM +0200, Christian König wrote:
> 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.

TPHS includes an bit of data on every TLP and the data transits to the
eventual completer.

It does not have to be a root port.
 
> For P2P that is obviously nonsense because we don't have P2P support
> for cached accesses.

For P2P the TPH data on the TLP will transit to the P2P completer
unchanged.

It is up to the completer do define what it does with the TPH data.

Typically root ports in CPUs will use TPH data for cache placement
instructions. But who knows what a P2P device will use it for.

In Linux the driver that owns the completing address space gets to
specify how the TPH data works based on its own device specific
knowledge.

Jason