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

[Jason Gunthorpe <jgg@nvidia.com>]

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.

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.

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

Jason