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

Leon Romanovsky
2025-11-19T20:45:46Z

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