linux-kernel - Re: [PATCH v8 05/11] PCI/P2PDMA: Document DMABUF model

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Message-ID: <20251119131313.GA18335@unreal>
Date: Wed, 19 Nov 2025 15:13:13 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Christian König <christian.koenig@....com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>, Jens Axboe <axboe@...nel.dk>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jason Gunthorpe <jgg@...pe.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Sumit Semwal <sumit.semwal@...aro.org>, Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Ankit Agrawal <ankita@...dia.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <skolothumtho@...dia.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex@...zbot.org>,
	Krishnakant Jaju <kjaju@...dia.com>, Matt Ochs <mochs@...dia.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, iommu@...ts.linux.dev,
	linux-mm@...ck.org, linux-doc@...r.kernel.org,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, kvm@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v8 05/11] PCI/P2PDMA: Document DMABUF model

On Wed, Nov 19, 2025 at 10:18:08AM +0100, Christian König wrote:
> 
> 
> On 11/11/25 10:57, Leon Romanovsky wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > 
> > Reflect latest changes in p2p implementation to support DMABUF lifecycle.
> > 
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> > Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> > ---
> >  Documentation/driver-api/pci/p2pdma.rst | 95 +++++++++++++++++++++++++--------
> >  1 file changed, 72 insertions(+), 23 deletions(-)

<...>

> > These MMIO pages have no struct page, and
> 
> Well please drop "pages" here. Just say MMIO addresses.
> 
> > +if used with mmap() must create special PTEs. As such there are very few
> > +kernel uAPIs that can accept pointers to them; in particular they cannot be used
> > +with read()/write(), including O_DIRECT.

<...>

> > +DMABUF provides an alternative to the above struct page-based
> > +client/provider/orchestrator system. In this mode the exporting driver will wrap
> > +some of its MMIO in a DMABUF and give the DMABUF FD to userspace.
> > +
> > +Userspace can then pass the FD to an importing driver which will ask the
> > +exporting driver to map it.
> 
> "to map it to the importer".

No problem, changed.

> 
> Regards,
> Christian.
> 
> > +
> > +In this case the initiator and target pci_devices are known and the P2P subsystem
> > +is used to determine the mapping type. The phys_addr_t-based DMA API is used to
> > +establish the dma_addr_t.
> > +
> > +Lifecycle is controlled by DMABUF move_notify(). When the exporting driver wants
> > +to remove() it must deliver an invalidation shutdown to all DMABUF importing
> > +drivers through move_notify() and synchronously DMA unmap all the MMIO.
> > +
> > +No importing driver can continue to have a DMA map to the MMIO after the
> > +exporting driver has destroyed its p2p_provider.
> >  
> >  
> >  P2P DMA Support Library
> > 
>