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

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

On 5/28/26 06:55, Zhiping Zhang wrote:
> On Wed, May 27, 2026 at 5:53 AM Christian König
> <christian.koenig@amd.com> wrote:
>>
>>>
>> On 5/27/26 14:36, Jason Gunthorpe wrote:
>>> On Wed, May 27, 2026 at 02:23:46PM +0200, Christian König wrote:
>>>
>>>> Yeah that's a good point, I should probably rephrase the question.
>>>>
>>>> I'm aware of how TPH works by adding the extra ST to the TLP.
>>>>
>>>> But my question is how is that useful to a PCIe endpoint? What is the effect of the ST here?
>>>
>>> TBH I've never heard Meta explain what their device is doing with
>>> it. At least it seems to be super important to their device..
>>
>> Yeah I think at least a brief description of what is going on here would be necessary for the review.
>>
>> Otherwise we have only the info that the exporter wants to give an opaque ST for the importer to use and no technical description what that is good for, how to test it etc...
>>
>> Regards,
>> Christian.
>>
>>>
>>> Jason
>>
> 
> Fair point — I'll add a couple of paragraphs to the v6 cover letter and the
> patch's changelog as well.
> 
> The short version: in this series the vfio-pci device is the completer
> of the P2P
> writes and mlx5 is the requester. As Jason noted, ST semantics on the completer
> are implementation-defined, so only the driver that owns the completer (here,
> vfio-pci on behalf of its userspace owner) can hand out a meaningful ST; the
> importer treats it as opaque and just places it on the TLP.

Yeah but that is not really sufficient to justify a driver 2 driver interface.

Which PF driver is backing the vfio-pci and what effect does sending TLPs with ST to it compared to TLPs without an ST?

Regards,
Christian.

> Validation occurs at two levels: PCIe analyzer captures on P2P TLPs, and the
>  end-to-end P2P workload yields only expected results.
> 
> Thanks,
> Zhiping