Re: [PATCH v2 03/10] PCI/P2PDMA: Refactor to separate core P2P functionality from memory allocation

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Sep 22, 2025 at 03:00:32PM -0600, Alex Williamson wrote:
> But then later in patch 8/ and again in 10/ why exactly do we cache
> the provider on the vfio_pci_core_device rather than ask for it on
> demand from the p2pdma?

It makes the most sense if the P2P is activated once during probe(),
it is just a cheap memory allocation, so no reason not to.

If you try to do it on-demand then it will require more locking.

> It also seems like the coordination of a valid provider is ad-hoc
> between p2pdma and vfio-pci.  For example, this only fills providers
> for MMIO BARs and vfio-pci validates that dmabuf operations are for
> MMIO BARs, but it would be more consistent if vfio-pci relied on p2pdma
> to give it a valid provider for a given BAR.  Thanks,

Yeah, validate_dmabuf_input() should check
priv->vdev->provider[priv->bar] for NULL and I think we should
directly store the non-NUL: provider in the dmabuf priv struct instead
of the bar index and replace these:

+       provider = priv->vdev->provider[priv->bar];
+       provider = priv->vdev->provider[priv->bar];

Jason