linux-kernel - RE: [PATCH v5 4/8] vfio: Wait for dma-buf invalidation to complete

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Message-ID: <BN9PR11MB5276B99D4E8C6496B0C447888C9EA@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 29 Jan 2026 07:06:37 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>
CC: Pranjal Shrivastava <praan@...gle.com>, Sumit Semwal
	<sumit.semwal@...aro.org>, Christian König
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	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
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Subject: RE: [PATCH v5 4/8] vfio: Wait for dma-buf invalidation to complete

> From: Jason Gunthorpe <jgg@...pe.ca>
> Sent: Wednesday, January 28, 2026 12:28 AM
> 
> On Tue, Jan 27, 2026 at 10:58:35AM +0200, Leon Romanovsky wrote:
> > > > @@ -333,7 +359,37 @@ 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_invalidate_mappings(priv->dmabuf);
> > > > +			dma_resv_wait_timeout(priv->dmabuf->resv,
> > > > +					      DMA_RESV_USAGE_BOOKKEEP,
> false,
> > > > +					      MAX_SCHEDULE_TIMEOUT);
> > > >  			dma_resv_unlock(priv->dmabuf->resv);
> > > > +			if (revoked) {
> > > > +				kref_put(&priv->kref,
> vfio_pci_dma_buf_done);
> > > > +				/* Let's wait till all DMA unmap are
> completed. */
> > > > +				wait = wait_for_completion_timeout(
> > > > +					&priv->comp, secs_to_jiffies(1));
> > >
> > > Is the 1-second constant sufficient for all hardware, or should the
> > > invalidate_mappings() contract require the callback to block until
> > > speculative reads are strictly fenced? I'm wondering about a case where
> > > a device's firmware has a high response latency, perhaps due to internal
> > > management tasks like error recovery or thermal and it exceeds the 1s
> > > timeout.
> > >
> > > If the device is in the middle of a large DMA burst and the firmware is
> > > slow to flush the internal pipelines to a fully "quiesced"
> > > read-and-discard state, reclaiming the memory at exactly 1.001 seconds
> > > risks triggering platform-level faults..
> > >
> > > Since the wen explicitly permit these speculative reads until unmap is
> > > complete, relying on a hardcoded timeout in the exporter seems to
> > > introduce a hardware-dependent race condition that could compromise
> > > system stability via IOMMU errors or AER faults.
> > >
> > > Should the importer instead be required to guarantee that all
> > > speculative access has ceased before the invalidation call returns?
> >
> > It is guaranteed by the dma_resv_wait_timeout() call above. That call
> ensures
> > that the hardware has completed all pending operations. The 1‑second
> delay is
> > meant to catch cases where an in-kernel DMA unmap call is missing, which
> should
> > not trigger any DMA activity at that point.
> 
> Christian may know actual examples, but my general feeling is he was
> worrying about drivers that have pushed the DMABUF to visibility on
> the GPU and the move notify & fences only shoot down some access. So
> it has to wait until the DMABUF is finally unmapped.
> 
> Pranjal's example should be covered by the driver adding a fence and
> then the unbounded fence wait will complete it.
> 

Bear me if it's an ignorant question.

The commit msg of patch6 says that VFIO doesn't tolerate unbounded
wait, which is the reason behind the 2nd timeout wait here.

Then why is "the unbounded fence wait" not a problem in the same
code path? the use of MAX_SCHEDULE_TIMEOUT imply a worst-case
timeout in hundreds of years...

and it'd be helpful to put some words in the code based on what's
discussed here.