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 15, 2025 at 12:21:05AM -0700, Vivek Kasireddy wrote:
> Typically, functions of the same PCI device (such as a PF and a VF)
> share the same bus and have a common root port and the PF provisions
> resources for the VF. Given this model, they can be considered
> compatible as far as P2PDMA access is considered.

Huh? I'm not sure I understand what this is about. Please be more
clear what your use case is and what exactly is not working.

If it is talking about internal loopback within a single function
between PF and VF, then no, this is very expressly not something that
should be expected to work by default!

In fact I would consider any SRIOV capable device that had such a
behavior by default to be catastrophically security broken.

So this patch can't be talking about that, right?

Yet that is what this code seems to do?!?!?

> +static bool pci_devfns_support_p2pdma(struct pci_dev *provider,
> +				      struct pci_dev *client)
> +{
> +	if (provider->vendor == PCI_VENDOR_ID_INTEL &&
> +	    client->vendor == PCI_VENDOR_ID_INTEL) {
> +		if ((pci_is_vga(provider) && pci_is_vga(client)) ||
> +		    (pci_is_display(provider) && pci_is_display(client)))
> +			return pci_physfn(provider) == pci_physfn(client);
> +	}

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.

If your device supports P2P between VF and PF then iommu grouping must
put VFs in the PF's group and you loose VFIO support.

Jason