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

[Jason Gunthorpe <jgg@ziepe.ca>]

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.

I think the open question here is if drivers that can't rely on their
fences should return dma_buf_attach_revocable() = false ? It depends
on how long they will leave the buffers mapped, and if it is "bounded
time".

The converse is we want to detect bugs where drivers have wrongly set
dma_buf_attach_revocable() = true and this turns into an infinite
sleep, so the logging is necessary, IMHO.

At worst the code should sleep 1s, print, then keep sleeping..

Jason