Re: [PATCH v4 1/5] PCI/P2PDMA: Don't enforce ACS check for device functions of Intel GPUs

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

On 22.09.25 14:20, Jason Gunthorpe wrote:
> On Mon, Sep 22, 2025 at 01:22:49PM +0200, Christian König wrote:
> 
>> Well what exactly is happening here? You have a PF assigned to the
>> host and a VF passed through to a guest, correct?
>>
>> And now the PF (from the host side) wants to access a BAR of the VF?
> 
> Not quite.
> 
> It is a GPU so it has a pool of VRAM. The PF can access all VRAM and
> the VF can access some VRAM.
> 
> They want to get a DMABUF handle for a bit of the VF's reachable VRAM
> that the PF can import and use through it's own funciton.

Yeah, where's the problem? We do that all the time.

> The use of the VF's BAR in this series is an ugly hack. The PF never
> actually uses the VF BAR, it just hackily converts the dma_addr_t back
> to CPU physical and figures out where it is in the VRAM pool and then
> uses a PF centric address for it.

Oh my, please absolutely don't do that!

If you have a device internal connection then you need special handling between your PF and VF driver.

But I still don't have the full picture of who is using what here, e.g. who is providing the DMA-buf handle and who is using it?

Regards,
Christian.

> 
> All they want is either the actual VRAM address or the CPU physical.
> 
> Jason