Re: [Qemu-devel] [PATCH for 2.8 10/11] Revert "intel_iommu: Throw hw_error on notify_started"

[Peter Xu]

On Wed, Aug 31, 2016 at 08:43:42PM -0600, Alex Williamson wrote:
> > > >>This reverts commit 3cb3b1549f5401dc3a5e1d073e34063dc274136f. Vhost
> > > >>device IOTLB API will get notified and send invalidation request to
> > > >>vhost through this notifier.  
> > > >AFAICT this series does not address the original problem for which
> > > >commit 3cb3b1549f54 was added.  We've only addressed the very narrow
> > > >use case of a device iotlb firing the iommu notifier therefore this
> > > >change is a regression versus 2.7 since it allows invalid
> > > >configurations with a physical iommu which will never receive the
> > > >necessary notifies from intel-iommu emulation to work properly.  Thanks,
> > > >
> > > >Alex  
> > > 
> > > Looking at vfio, it cares about map but vhost only cares about IOTLB
> > > invalidation. Then I think we probably need another kind of notifier in 
> > > this
> > > case to avoid this.  
> > 
> > Shall we leverage IOMMUTLBEntry.perm == IOMMU_NONE as a sign for
> > invalidation? If so, we can use the same IOTLB interface as before.
> > IMHO these two interfaces are not conflicting?
> > 
> > Alex,
> > 
> > Do you mean we should still disallow user from passing through devices
> > while Intel IOMMU enabled? If so, not sure whether patch below can
> > solve the issue.
> > 
> > It seems that we need a "name" for either IOMMU notifier
> > provider/consumer, and we should not allow (provider==Intel &&
> > consumer==VFIO) happen. In the following case, I added a name for
> > provider, and VFIO checks it.
> 
> Absolutely not, intel-iommu emulation is simply incomplete, the IOMMU
> notifier is never called for mappings.  There's a whole aspect of
> iommu notifiers that intel-iommu simply hasn't bothered to implement.
> Don't punish vfio for actually making use of the interface as it was
> intended to be used.  AFAICT you're implementing the unmap/invalidation
> half, without the actual mapping half of the interface.  It's broken
> and incompatible with any iommu notifiers that expect to see both
> sides.  Thanks,

Yeah I think I got your point. Thanks for the explanation.

Now I agree with Jason that we may need another notifier mechanism.

-- peterx