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

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Message-ID: <20251119204541.GN18335@unreal>
Date: Wed, 19 Nov 2025 22:45:41 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Christian König <christian.koenig@....com>,
	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>,
	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 03:45:06PM -0400, Jason Gunthorpe wrote:
> On Wed, Nov 19, 2025 at 03:06:18PM +0100, Christian König wrote:
> > On 11/19/25 14:35, Jason Gunthorpe wrote:
> > > On Wed, Nov 19, 2025 at 10:18:08AM +0100, Christian König wrote:
> > >>> +As this is not well-defined or well-supported in real HW the kernel defaults to
> > >>> +blocking such routing. There is an allow list to allow detecting known-good HW,
> > >>> +in which case P2P between any two PCIe devices will be permitted.

<...>

> > The documentation makes it sound like DMA-buf is limited to not
> > using struct pages and direct I/O, but that is not true.
> 
> Okay, I see what you mean, the intention was to be very strong and say
> if you are not using struct pages then you must using DMABUF or
> something like it to control lifetime. Not to say that was the only
> way how DMABUF can be used.
> 
> Leon let's try to clarify that a bit more

diff --git a/Documentation/driver-api/pci/p2pdma.rst b/Documentation/driver-api/pci/p2pdma.rst
index 32e9b691508b..280673b50350 100644
--- a/Documentation/driver-api/pci/p2pdma.rst
+++ b/Documentation/driver-api/pci/p2pdma.rst
@@ -156,7 +156,8 @@ Usage With DMABUF
 =================
 
 DMABUF provides an alternative to the above struct page-based
-client/provider/orchestrator system. In this mode the exporting driver will wrap
+client/provider/orchestrator system and should be used when struct page
+doesn't exist. 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


> 
> Jason