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

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Message-ID: <20250923174333.GE2608121@nvidia.com>
Date: Tue, 23 Sep 2025 14:43:33 -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 Tue, Sep 23, 2025 at 11:30:41AM -0600, Alex Williamson wrote:
> On Tue, 23 Sep 2025 12:04:14 -0300
> Jason Gunthorpe <jgg@...dia.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.

I agree it is a weird interface, but it is close to the existing weird
interface :\

> 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.  

It uses devm to call pci_p2pdma_release() which NULL's pdev->p2pdma.

Jason