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

Alex Williamson
2025-09-23T17:30:48Z

On Tue, 23 Sep 2025 12:04:14 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> 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.

I'm only wondering about splitting to an "initialize/setup" function
where providers for each BAR are setup, and a "get provider" interface,
which doesn't really seem to be a hot path anyway. Batching could
still be done to setup all BAR providers at once.

However, the setup isn't really once per probe(), even in the case of a
new driver probing we re-use the previously setup providers. Doesn't
that introduce a problem that the provider bus_offset can be stale if
something like a BAR resize has occurred between drivers?

Possibly the providers should be setup in PCI core, a re-init triggered
for resource updates, and the driver interface is only to get the
provider. Thanks,

Alex