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

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Message-ID: <20250923150414.GA2608121@nvidia.com>
Date: Tue, 23 Sep 2025 12:04:14 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Leon Romanovsky <leon@...nel.org>, Leon Romanovsky <leonro@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	kvm@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linux-pci@...r.kernel.org, Logan Gunthorpe <logang@...tatee.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 03/10] PCI/P2PDMA: Refactor to separate core P2P
 functionality from memory allocation

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