Re: [Qemu-devel] [PATCH v3 2/3] memory: generalize iommu_ops.notify_started to notifier_add

[Paolo Bonzini]

On 07/09/2016 08:05, David Gibson wrote:
> On Wed, Sep 07, 2016 at 01:32:23PM +0800, Peter Xu wrote:
>> Considering that we may have multiple IOMMU notifier consumers in the
>> future, converting iommu_ops.notify_{started|stopped} into some more
>> general form. Now we can trap all notifier registerations and
>> deregistrations, rather than only the first ones.
>>
>> Power was leveraging the notifier_{started|stopped}, adding iommu_user
>> field for counting on Power guests to achieve the same goal.
> 
> Requiring each vIOMMU frontend to reference count or whatever seems
> like a pain.  The semantics of notify_started() were designed to avoid
> that.
> 
> Instead I'd suggest a callback which gets tripped any time the logical
> OR of the requested notifications for all current notifiers changes.

I like the suggestion.  Alternatively you could call notify_stopped if
old & ~new is nonzero (and you pass old & ~new), and notify_started if
new & ~old is nonzero (and you pass new & ~old).

Paolo