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

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Message-ID: <20260130140047.GD2328995@ziepe.ca>
Date: Fri, 30 Jan 2026 10:00:47 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christian König <christian.koenig@....com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	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 Fri, Jan 30, 2026 at 09:43:22AM +0100, Christian König wrote:
> On 1/24/26 20:14, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > Some exporters need a flow to synchronously revoke access to the DMA-buf
> > by importers. Once revoke is completed the importer is not permitted to
> > touch the memory otherwise they may get IOMMU faults, AERs, or worse.
> 
> That approach is seriously not going to fly.
> 
> You can use the invalidate_mappings approach to trigger the importer
> to give back the mapping, but when the mapping is really given back
> is still completely on the importer side.

Yes, and that is what this is all doing, there is the wait for the
importer's unmap to happen in the sequence.

> In other words you can't do the shot down revoke semantics you are
> trying to establish here.

All this is doing is saying if dma_buf_attach_revocable() == true then
the importer will call unmap within bounded time after
dma_buf_invalidate_mappings().

That's it. If the importing driver doesn't want to do that then it
should make dma_buf_attach_revocable()=false.

VFIO/etc only want to interwork with importers that can do this.

Jason