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

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Message-ID: <20250926081350.16bb66c8.alex.williamson@redhat.com>
Date: Fri, 26 Sep 2025 08:13:50 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Leon Romanovsky <leon@...nel.org>, 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 Thu, 25 Sep 2025 20:02:36 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Thu, Sep 25, 2025 at 04:31:31PM -0600, Alex Williamson wrote:
> > On Thu, 25 Sep 2025 08:53:08 -0300
> > Jason Gunthorpe <jgg@...dia.com> wrote:
> >   
> > > On Thu, Sep 25, 2025 at 10:03:14AM +0300, Leon Romanovsky wrote:
> > >   
> > > > > 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.    
> > > > 
> > > > I'll change it now. If "enable" function goes to be "get" function, we
> > > > won't need to store anything in vfio_pci_dma_buf too. At the end, we
> > > > have exactly two lines "provider = priv->vdev->provider[priv->bar];",
> > > > which can easily be changed to be "provider = pcim_p2pdma_provider(priv->vdev->pdev, priv->bar)"    
> > > 
> > > Not without some kind of locking change. I'd keep the
> > > priv->vdev->provider[priv->bar] because setup during probe doesn't
> > > need special locking.  
> > 
> > Why do we need to store the provider on the vfio_pci_core_device at
> > probe though, we can get it later via pcim_p2pdma_provider().   
> 
> Because you'd need some new locking to prevent races.

The race is avoided if we simply call pcim_p2pdma_provider() during
probe.  We don't need to save the returned provider.  That's where it
seems like pulling the setup out to a separate function would eliminate
this annoying BAR# arg.
 
> Besides, the model here should be to call the function once during
> probe and get back the allocated provider. The fact internally it is
> kind of nutzo still shouldn't leak out as a property of the ABI.
> 
> I would like to remove this weird behavior where it caches things
> inside the struct device. That's not normal for an API to do that, it
> is only done for the genalloc path that this doesn't use.

My goal in caching the provider on the vfio p2pdma object was to avoid
caching it on the vfio_pci_core_device, but now we're storing it on the
struct device, the vfio_pci_core_device, AND the vfio p2pdma object.
Given the current state that it's stored on the struct device, I think
we only need a setup call during probe (that could be stubbed out
rather than #ifdef'd), then cache the provider on the vfio p2pdma
object when a dmabuf is configured.  Thanks,

Alex