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

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Message-ID: <20260121075929.GU13201@unreal>
Date: Wed, 21 Jan 2026 09:59:29 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Matthew Brost <matthew.brost@...el.com>
Cc: 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>,
	Jason Gunthorpe <jgg@...pe.ca>, 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>,
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	Ankit Agrawal <ankita@...dia.com>,
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	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
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Subject: Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

On Tue, Jan 20, 2026 at 12:44:50PM -0800, Matthew Brost wrote:
> On Tue, Jan 20, 2026 at 04:07:06PM +0200, 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>
> > ---
> >  drivers/vfio/pci/vfio_pci_dmabuf.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > index d4d0f7d08c53..33bc6a1909dd 100644
> > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > @@ -321,6 +321,9 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
> >  			dma_resv_lock(priv->dmabuf->resv, NULL);
> >  			priv->revoked = revoked;
> >  			dma_buf_move_notify(priv->dmabuf);
> > +			dma_resv_wait_timeout(priv->dmabuf->resv,
> > +					      DMA_RESV_USAGE_KERNEL, false,
> > +					      MAX_SCHEDULE_TIMEOUT);
> 
> Should we explicitly call out in the dma_buf_move_notify() /
> invalidate_mappings kernel-doc that KERNEL slots are the mechanism
> for communicating asynchronous dma_buf_move_notify /
> invalidate_mappings events via fences?
> 
> Yes, this is probably implied, but it wouldn’t hurt to state this
> explicitly as part of the cross-driver contract.
> 
> Here is what we have now:
> 
>  	 * - Dynamic importers should set fences for any access that they can't
> 	 *   disable immediately from their &dma_buf_attach_ops.invalidate_mappings
>  	 *   callback.

I believe I documented this in patch 4:
https://lore.kernel.org/all/20260120-dmabuf-revoke-v3-4-b7e0b07b8214@nvidia.com/"
Is there anything else that should be added?

  1275 /**
  1276  * dma_buf_move_notify - notify attachments that DMA-buf is moving
  1277  *
  1278  * @dmabuf:     [in]    buffer which is moving
  1279  *
  1280  * Informs all attachments that they need to destroy and recreate all their
  1281  * mappings. If the attachment is dynamic then the dynamic importer is expected
  1282  * to invalidate any caches it has of the mapping result and perform a new
  1283  * mapping request before allowing HW to do any further DMA.
  1284  *
  1285  * If the attachment is pinned then this informs the pinned importer that
  1286  * the underlying mapping is no longer available. Pinned importers may take
  1287  * this is as a permanent revocation so exporters should not trigger it
  1288  * lightly.
  1289  *
  1290  * For legacy pinned importers that cannot support invalidation this is a NOP.
  1291  * Drivers can call dma_buf_attach_revocable() to determine if the importer
  1292  * supports this.
  1293  *
  1294  * NOTE: The invalidation triggers asynchronous HW operation and the callers
  1295  * need to wait for this operation to complete by calling
  1296  * to dma_resv_wait_timeout().
  1297  */

Thanks


> 
> Matt
> 
> >  			dma_resv_unlock(priv->dmabuf->resv);
> >  		}
> >  		fput(priv->dmabuf->file);
> > @@ -342,6 +345,8 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
> >  		priv->vdev = NULL;
> >  		priv->revoked = true;
> >  		dma_buf_move_notify(priv->dmabuf);
> > +		dma_resv_wait_timeout(priv->dmabuf->resv, DMA_RESV_USAGE_KERNEL,
> > +				      false, MAX_SCHEDULE_TIMEOUT);
> >  		dma_resv_unlock(priv->dmabuf->resv);
> >  		vfio_device_put_registration(&vdev->vdev);
> >  		fput(priv->dmabuf->file);
> > 
> > -- 
> > 2.52.0
> >