Re: [v4 2/3] vfio_register_notifier: also register on the group notifier

[Jike Song <jike.song@intel.com>]

On 11/17/2016 02:03 PM, Alex Williamson wrote:
> On Thu, 17 Nov 2016 13:24:59 +0800
> Jike Song <jike.song@intel.com> wrote:
> 
>> On 11/17/2016 03:45 AM, Alex Williamson wrote:
>>> Perhaps calling it a filter is not correct, I was thinking that a
>>> vendor driver would register the notifier with a set of required
>>> events.  The driver would need to handle/ignore additional events
>>> outside of the required mask.  There are certainly some complications
>>> to this model though that I hadn't thought all the way through until
>>> now.  For instance what if we add event XYZ in the future and the
>>> vendor driver adds this to their required mask.  If we run that on an
>>> older kernel where the vfio infrastructure doesn't know about that
>>> event, the vendor driver needs to fail, or at least know that event is
>>> not supported and retry with a set of supported events.
>>>
>>> There's another problem with my proposal too, we can't put a single
>>> notifier_block on multiple notifier_block heads, that just doesn't
>>> work.  So we probably need to separate a group notifier from an iommu
>>> notifier, the vendor driver will need to register for each.
>>>
>>> Maybe we end up with something like:
>>>
>>> int vfio_register_notifier(struct device *dev,
>>> 			   vfio_notify_type_t type,
>>> 			   unsigned long *required_events,
>>> 			   struct notifier_block *nb);
>>>
>>> typedef unsigned short vfio_notify_type_t;
>>> enum vfio_notify_type {
>>> 	VFIO_IOMMU_NOTIFY = (__force vfio_notify_type_t)0,
>>> 	VFIO_GROUP_NOTIFY = (__force vfio_notify_type_t)1,
>>> };
>>>
>>> (stealing this from pci_dev_flags_t, hope it works)
>>>
>>> A VFIO_GROUP_NOTIFY would add the notifier_block to the vfio_group, a
>>> VFIO_IOMMU_NOTIFY would add it to vfio_iommu.  Each would have their
>>> own unique set of events and each would compare their supported events
>>> to the required events by the caller.  Supported events would be
>>> cleared from the callers required_events arg.  If required_events still
>>> has bits set, the notifier_block is not registered, an error is
>>> returned, and the caller can identify the unsupported events by the
>>> remaining bits in the required_events arg.  Can that work?  Thanks,  
>>
>> Let me summarize the discussion:
>>
>> - There should be 2 notifier heads, one in vfio_iommu another in vfio_group;
>> - vfio_{un}register_notifier() has the type specified in parameter
>> - In vfio_register_notifier, maybe:
>>
>> 	static vfio_iommu_register_notifier() {..}
>> 	static vfio_group_register_notifier() {..}
>> 	int vfio_register_notififer(type..
>> 	{
>> 		if (type == VFIO_IOMMU_NOTIFY)
>> 			return vfio_iommu_register_notifier();
>> 		if (type == VFIO_GROUP_NOTIFY)
>> 			return vfio_group_register_notifier();
>> 	}
>>
>>
>>
>> What's more, if we still want registration to be done in mdev framework,
>> we should change parent_ops:
>>
>> - rename 'notifier' to 'iommu_notifier'
>> - add "group_notifier"
>> - Add a group_events and a iommu_events to indicate what events vendor is
>>   interested in, respectively
>>
>> or otherwise don't touch it from mdev framework at all?
> 
> I think we should remove the notifier from the mdev framework and have
> the vendor drivers call vfio_{un}register_notifier() directly.  Note:
> 
>  - vfio_group_release() should be modified to remove any notifier
>    blocks remaining to prevent a stale call chain for the next user.

vfio_group_release calls vfio_group_unlock_and_free, which in turn calls 
kfree(group), so I guess a WARN_ON(group->notifier.head) before kfree
is enough?

>  - vfio_sanity_check_pfn_list() should be modified to WARN_ON remaining
>    notifier blocks on the vfio_iommu (ie. vendor drivers will need to
>    actively remove iommu notifiers since the vfio_iommu can persist
>    beyond the attachment of the mdev group, the WARN_ON will promote a
>    proactive approach to surfacing such issues).

I guess Kirti will prefer to pick up this? if not I also can do it :-)

> I'd like to get Kirti's current series in linux-next ASAP, so please
> submit a follow-on series to make these changes.  I hope we can get
> that finalized and added on top of Kirti's series before the v4.10
> merge window opens. Thanks,

Yes, I'll send out the follow-on series ASAP, since we also have KVMGT
depending on it to get notified by vfio...


--
Thanks,
Jike