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

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Message-ID: <20250923120932.47df57b2.alex.williamson@redhat.com>
Date: Tue, 23 Sep 2025 12:09:32 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.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, 23 Sep 2025 14:43:33 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> 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 :\

Seems like it would help if we just positioned it as a "get provider
for BAR" function that happens to initialize all the providers on the
first call, rather than an "enable" function with some strange BAR
argument and provider return.  pcim_p2pdma_provider(pdev, bar)?

It would at least make sense to me then to store the provider on the
vfio_pci_dma_buf object at the time of the get feature call rather than
vfio_pci_core_init_dev() though.  That would eliminate patch 08/ and
the inline #ifdefs.

> > 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.

Ah, right.  So the /* PCI device was "rebound" to the driver */ comment
is further misleading, a new probe would do a new setup.  Thanks,

Alex