linux-kernel - Re: [PATCH v4 07/10] vfio/pci: Add dma-buf export config for MMIO regions

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Message-ID: <20250930100758.1605d5a5.alex.williamson@redhat.com>
Date: Tue, 30 Sep 2025 10:07:58 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Jason Gunthorpe <jgg@...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 v4 07/10] vfio/pci: Add dma-buf export config for MMIO
 regions

On Tue, 30 Sep 2025 10:57:48 +0300
Leon Romanovsky <leon@...nel.org> wrote:

> On Mon, Sep 29, 2025 at 03:17:40PM -0600, Alex Williamson wrote:
> > On Sun, 28 Sep 2025 17:50:17 +0300
> > Leon Romanovsky <leon@...nel.org> wrote:
> >   
> > > From: Leon Romanovsky <leonro@...dia.com>
> > > 
> > > Add new kernel config which indicates support for dma-buf export
> > > of MMIO regions, which implementation is provided in next patches.
> > > 
> > > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> > > ---
> > >  drivers/vfio/pci/Kconfig | 20 ++++++++++++++++++++
> > >  1 file changed, 20 insertions(+)
> > > 
> > > diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> > > index 2b0172f54665..55ae888bf26a 100644
> > > --- a/drivers/vfio/pci/Kconfig
> > > +++ b/drivers/vfio/pci/Kconfig
> > > @@ -55,6 +55,26 @@ config VFIO_PCI_ZDEV_KVM
> > >  
> > >  	  To enable s390x KVM vfio-pci extensions, say Y.
> > >  
> > > +config VFIO_PCI_DMABUF
> > > +	bool "VFIO PCI extensions for DMA-BUF"
> > > +	depends on VFIO_PCI_CORE
> > > +	depends on PCI_P2PDMA && DMA_SHARED_BUFFER
> > > +	default y
> > > +	help
> > > +	  Enable support for VFIO PCI extensions that allow exporting
> > > +	  device MMIO regions as DMA-BUFs for peer devices to access via
> > > +	  peer-to-peer (P2P) DMA.
> > > +
> > > +	  This feature enables a VFIO-managed PCI device to export a portion
> > > +	  of its MMIO BAR as a DMA-BUF file descriptor, which can be passed
> > > +	  to other userspace drivers or kernel subsystems capable of
> > > +	  initiating DMA to that region.
> > > +
> > > +	  Say Y here if you want to enable VFIO DMABUF-based MMIO export
> > > +	  support for peer-to-peer DMA use cases.
> > > +
> > > +	  If unsure, say N.
> > > +
> > >  source "drivers/vfio/pci/mlx5/Kconfig"
> > >  
> > >  source "drivers/vfio/pci/hisilicon/Kconfig"  
> > 
> > This is only necessary if we think there's a need to build a kernel with
> > P2PDMA and VFIO_PCI, but not VFIO_PCI_DMABUF.  Does that need really
> > exist?  
> 
> It is used to filter build of vfio_pci_dmabuf.c - drivers/vfio/pci/Makefile:
> vfio-pci-core-$(CONFIG_VFIO_PCI_DMABUF) += vfio_pci_dmabuf.o

Maybe my question of whether it needs to exist at all is too broad.
Does it need to be a user visible Kconfig option?  Where do we see the
need to preclude this feature from vfio-pci if the dependencies are
enabled?

> > I also find it unusual to create the Kconfig before adding the
> > supporting code.  Maybe this could be popped to the end or rolled into
> > the last patch if we decided to keep it.  Thanks,  
> 
> It is leftover from previous version, I can squash it, but first we need
> to decide what to do with pcim_p2pdma_init() call, if it needs to be
> guarded or not.

As in the other thread, I think it would be cleaner in an IS_ENABLED
branch.  I'm tempted to suggest we filter out EOPNOTSUPP to allow it to
be unconditional, but I understand your point with the list_head
initialization.  Thanks,

Alex