linux-kernel - Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

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Message-ID: <20260121133146.GY961572@ziepe.ca>
Date: Wed, 21 Jan 2026 09:31:46 -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>,
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	Alex Williamson <alex@...zbot.org>,
	Ankit Agrawal <ankita@...dia.com>,
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Subject: Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

On Wed, Jan 21, 2026 at 10:20:51AM +0100, Christian König wrote:
> On 1/20/26 15:07, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > dma-buf invalidation is performed asynchronously by hardware, so VFIO must
> > wait until all affected objects have been fully invalidated.
> > 
> > Fixes: 5d74781ebc86 ("vfio/pci: Add dma-buf export support for MMIO regions")
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> 
> Reviewed-by: Christian König <christian.koenig@....com>
> 
> Please also keep in mind that the while this wait for all fences for
> correctness you also need to keep the mapping valid until
> dma_buf_unmap_attachment() was called.

Can you elaborate on this more?

I think what we want for dma_buf_attach_revocable() is the strong
guarentee that the importer stops doing all access to the memory once
this sequence is completed and the exporter can rely on it. I don't
think this works any other way.

This is already true for dynamic move capable importers, right?

For the non-revocable importers I can see the invalidate sequence is
more of an advisory thing and you can't know the access is gone until
the map is undone.

> In other words you can only redirect the DMA-addresses previously
> given out into nirvana (or a dummy memory or similar), but you still
> need to avoid re-using them for something else.

Does any driver do this? If you unload/reload a GPU driver it is
going to re-use the addresses handed out?

Jason