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

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/09/2015 06:33, 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>

The patch is okay, just two questions:

1) Is there a case where using the no-replay functions makes sense?

2) You could add a ->replay function to the iommu_ops to optimize it, if
you want.

Paolo