Re: [Qemu-devel] [PATCH 5/7] memory: Allow replay of IOMMU mapping notifications

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Sep 24, 2015 at 11:32:29AM -0600, Alex Williamson wrote:
> On Thu, 2015-09-24 at 14:33 +1000, David Gibson wrote:
> > When we have guest visible IOMMUs, we allow notifiers to be registered
> > which will be informed of all changes to IOMMU mappings.  This is used by
> > vfio to keep the host IOMMU mappings in sync with guest IOMMU mappings.
> > 
> > However, unlike with a memory region listener, an iommu notifier won't be
> > told about any mappings which already exist in the (guest) IOMMU at the
> > time it is registered.  This can cause problems if hotplugging a VFIO
> > device onto a guest bus which had existing guest IOMMU mappings, but didn't
> > previously have an VFIO devices (and hence no host IOMMU mappings).
> > 
> > This adds a memory_region_register_iommu_notifier_replay() function to
> > handle this case.  As well as registering the new notifier it replays
> > existing mappings.  Because the IOMMU memory region doesn't internally
> > remember the granularity of the guest IOMMU it has a small hack where the
> > caller must specify a granularity at which to replay mappings.
> > 
> > If there are finer mappings in the guest IOMMU these will be reported in
> > the iotlb structures passed to the notifier which it must handle (probably
> > causing it to flag an error).  This isn't new - the VFIO iommu notifier
> > must already handle notifications about guest IOMMU mappings too short
> > for it to represent in the host IOMMU.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

[snip]
> > +void memory_region_register_iommu_notifier_replay(MemoryRegion *mr, Notifier *n,
> > +                                                  hwaddr granularity,
> > +                                                  bool is_write)
> > +{
> > +    hwaddr addr;
> > +    IOMMUTLBEntry iotlb;
> > +
> > +    memory_region_register_iommu_notifier(mr, n);
> > +
> > +    for (addr = 0; addr < memory_region_size(mr); addr += granularity) {
> > +
> > +        iotlb = mr->iommu_ops->translate(mr, addr, is_write);
> > +        if (iotlb.perm != IOMMU_NONE) {
> > +            n->notify(n, &iotlb);
> > +        }
> > +    }
> > +}
> > +
> 
> 
> When memory_listener_register() replays mappings, it does so on an rcu
> copy of the flatview for each AddressSpace.  Here we don't seem to have
> anything protecting against concurrency... do we need to worry about
> that?

I was assuming that the IOMMU mappings are protected by the BQL.  I
_think_ that's the case (for every IOMMU we have so far), but I'm not
really sure how to be sure.

> 
> >  void memory_region_unregister_iommu_notifier(Notifier *n)
> >  {
> >      notifier_remove(n);
> 
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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