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

[Leon Romanovsky <leon@kernel.org>]

On Tue, Sep 23, 2025 at 12:09:32PM -0600, Alex Williamson wrote:
> On Tue, 23 Sep 2025 14:43:33 -0300
> Jason Gunthorpe <jgg@nvidia.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@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.  
> > 
> > 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.

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)"

> 
> > > 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,

I will fix the comment.

Thanks

> 
> Alex
>