linux-kernel - Re: [PATCH v5 6/8] dma-buf: Add dma_buf_attach

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Message-ID: <20260126210108.GD1641016@ziepe.ca>
Date: Mon, 26 Jan 2026 17:01:08 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Pranjal Shrivastava <praan@...gle.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Christian König <christian.koenig@....com>,
	Alex Deucher <alexander.deucher@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Felix Kuehling <Felix.Kuehling@....com>,
	Alex Williamson <alex@...zbot.org>,
	Ankit Agrawal <ankita@...dia.com>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	amd-gfx@...ts.freedesktop.org, virtualization@...ts.linux.dev,
	intel-xe@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, kvm@...r.kernel.org
Subject: Re: [PATCH v5 6/8] dma-buf: Add dma_buf_attach_revocable()

On Mon, Jan 26, 2026 at 08:38:44PM +0000, Pranjal Shrivastava wrote:
> I noticed that Patch 5 removes the invalidate_mappings stub from 
> umem_dmabuf.c, effectively making the callback NULL for an RDMA 
> importer. Consequently, dma_buf_attach_revocable() (introduced here)
> will return false for these importers.

Yes, that is the intention.

> Since the cover letter mentions that VFIO will use
> dma_buf_attach_revocable() to prevent unbounded waits, this appears to
> effectively block paths like the VFIO-export -> RDMA-import path..

It remains usable with the ODP path and people are using that right
now.

> Given that RDMA is a significant consumer of dma-bufs, are there plans
> to implement proper revocation support in the IB/RDMA core (umem_dmabuf)? 

This depends on each HW, they need a way to implement the revoke
semantic. I can't guess what is possible, but I would hope that most
HW could at least do a revoke on a real MR.

Eg a MR rereg operation to a kernel owned empty PD is an effective
"revoke", and MR rereg is at least defined by standards so HW should
implement it.
 
> It would be good to know if there's a plan for bringing such importers
> into compliance with the new revocation semantics so they can interop
> with VFIO OR are we completely ruling out users like RDMA / IB importing
> any DMABUFs exported by VFIO?

It will be driver dependent, there is no one shot update here.

Jason