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

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Message-ID: <20260121104451.GB13201@unreal>
Date: Wed, 21 Jan 2026 12:44:51 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Christian König <christian.koenig@....com>
Cc: Matthew Brost <matthew.brost@...el.com>,
	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>,
	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>,
	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,
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Subject: Re: [PATCH v3 6/7] vfio: Wait for dma-buf invalidation to complete

On Wed, Jan 21, 2026 at 11:41:48AM +0100, Christian König wrote:
> On 1/20/26 21:44, 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?
> 
> Oh, I missed that! And no that is not correct.
> 
> This should be DMA_RESV_USAGE_BOOKKEEP so that we wait for everything.

Will change.

> 
> Regards,
> Christian.
> 
> > 
> > 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.
> > 
> > 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
> >>
> 
>