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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 18, 2025 at 06:16:38AM +0000, Kasireddy, Vivek wrote:

> However, assuming that pci_p2pdma_map_type() did not fail, based on my
> experiments, the GPU PF is still unable to access the buffer located in VF's
> VRAM portion directly because it is represented using PCI BAR addresses.

In this case messing with ACS is completely wrong. If the intention is
to convay a some kind of "private" address representing the physical
VRAM then you need to use a DMABUF mechanism to do that, not deliver a
P2P address that the other side cannot access.

Christian told me dmabuf has such a private address mechanism, so
please figure out a way to use it..

> > Do not open code quirks like this in random places, if this device
> > supports some weird ACS behavior and does not include it in the ACS
> > Caps the right place is to supply an ACS quirk in quirks.c so all the
> > code knows about the device behavior, including the iommu grouping.
> Ok, I'll move it to quirks.c.

No, don't, it is completely wrong to mess with ACS flags for the
problem you are trying to solve.

> On my test system, it looks like the VFs and the PF are put into different
> iommu groups. I am checking with our hardware folks to understand how this
> is expected to work but does it mean that P2P between PF and VF is not
> supported in my case?

A special internal path through VRAM is outside the scope of iommu
grouping.

Jason