Re: [v3 4/5] vfio: implement APIs to set/put kvm to/from vfio group

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

On 11/10/2016 12:10 PM, Jike Song wrote:
> On 11/10/2016 01:53 AM, Alex Williamson wrote:
>> On Wed, 09 Nov 2016 20:49:32 +0800
>> Jike Song <jike.song@intel.com> wrote:
>>
>>> On 11/08/2016 04:45 AM, Paolo Bonzini wrote:
>>>> On 07/11/2016 19:28, Alex Williamson wrote:  
>>>>>>> Can the reference become invalid?    
>>>>>>
>>>>>> No, this is guaranteed by virt/kvm/vfio.c + the udata.lock mutex (which
>>>>>> probably should be renamed...).  
>>>>>
>>>>> The caller gets a reference to kvm, but there's no guarantee that the
>>>>> association of that kvm reference to the group stays valid.  Once we're
>>>>> outside of that mutex, we might as well consider that kvm:group
>>>>> association stale.
>>>>>    
>>>>>>> The caller may still hold
>>>>>>> a kvm references, but couldn't the group be detached from one kvm
>>>>>>> instance and re-attached to another?    
>>>>>>
>>>>>> Can this be handled by the vendor driver?  Does it get a callback when
>>>>>> it's detached from a KVM instance?  
>>>>>
>>>>> The only release callback through vfio is when the user closes the
>>>>> device, the code in this series is the full extent of vfio awareness of
>>>>> kvm.  Thanks,  
>>>>
>>>> Maybe there should be an mdev callback at the point of association and
>>>> deassociation between VFIO and KVM.  Then the vendor driver can just use
>>>> the same mutex for association, deassociation and usage.  I'm not even
>>>> sure that these patches are necessary once you have that callback.  
>>>
>>> Hi Alex & Paolo,
>>>
>>> So I cooked another draft version of this, there is no kvm pointer saved
>>> in vfio_group in this version, and notifier will be called on attach/detach,
>>> please kindly have a look :-)
>>>
>>>
>>> --
>>> Thanks,
>>> Jike
>>>
>>>
>>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
>>> index ed2361e4..20b5da9 100644
>>> --- a/drivers/vfio/vfio.c
>>> +++ b/drivers/vfio/vfio.c
>>> @@ -34,6 +34,7 @@
>>>  #include <linux/uaccess.h>
>>>  #include <linux/vfio.h>
>>>  #include <linux/wait.h>
>>> +#include <linux/kvm_host.h>
>>>  
>>>  #define DRIVER_VERSION	"0.3"
>>>  #define DRIVER_AUTHOR	"Alex Williamson <alex.williamson@redhat.com>"
>>> @@ -86,6 +87,10 @@ struct vfio_group {
>>>  	struct mutex			unbound_lock;
>>>  	atomic_t			opened;
>>>  	bool				noiommu;
>>> +	struct {
>>> +		struct mutex lock;
>>> +		struct blocking_notifier_head notifier;
>>> +	} udata;
>>>  };
>>>  
>>>  struct vfio_device {
>>> @@ -333,6 +338,7 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group)
>>>  	mutex_init(&group->device_lock);
>>>  	INIT_LIST_HEAD(&group->unbound_list);
>>>  	mutex_init(&group->unbound_lock);
>>> +	mutex_init(&group->udata.lock);
>>>  	atomic_set(&group->container_users, 0);
>>>  	atomic_set(&group->opened, 0);
>>>  	group->iommu_group = iommu_group;
>>> @@ -414,10 +420,11 @@ static void vfio_group_release(struct kref *kref)
>>>  	iommu_group_put(iommu_group);
>>>  }
>>>  
>>> -static void vfio_group_put(struct vfio_group *group)
>>> +void vfio_group_put(struct vfio_group *group)
>>>  {
>>>  	kref_put_mutex(&group->kref, vfio_group_release, &vfio.group_lock);
>>>  }
>>> +EXPORT_SYMBOL_GPL(vfio_group_put);
>>>  
>>>  /* Assume group_lock or group reference is held */
>>>  static void vfio_group_get(struct vfio_group *group)
>>> @@ -480,7 +487,7 @@ static struct vfio_group *vfio_group_get_from_minor(int minor)
>>>  	return group;
>>>  }
>>>  
>>> -static struct vfio_group *vfio_group_get_from_dev(struct device *dev)
>>> +struct vfio_group *vfio_group_get_from_dev(struct device *dev)
>>>  {
>>>  	struct iommu_group *iommu_group;
>>>  	struct vfio_group *group;
>>> @@ -494,6 +501,7 @@ static struct vfio_group *vfio_group_get_from_dev(struct device *dev)
>>>  
>>>  	return group;
>>>  }
>>> +EXPORT_SYMBOL_GPL(vfio_group_get_from_dev);
>>>  
>>>  /**
>>>   * Device objects - create, release, get, put, search
>>> @@ -1745,6 +1753,44 @@ long vfio_external_check_extension(struct vfio_group *group, unsigned long arg)
>>>  }
>>>  EXPORT_SYMBOL_GPL(vfio_external_check_extension);
>>>  
>>> +int vfio_group_register_notifier(struct vfio_group *group, struct notifier_block *nb)
>>> +{
>>> +	return blocking_notifier_chain_register(&group->udata.notifier, nb);
>>> +}
>>> +EXPORT_SYMBOL_GPL(vfio_group_register_notifier);
>>> +
>>> +int vfio_group_unregister_notifier(struct vfio_group *group, struct notifier_block *nb)
>>> +{
>>> +	return blocking_notifier_chain_unregister(&group->udata.notifier, nb);
>>> +}
>>> +EXPORT_SYMBOL_GPL(vfio_group_unregister_notifier);
>>
>> Kirti is already adding vfio_register_notifier &
>> vfio_unregister_notifier, these are not exclusive to the iommu, I
>> clarified that in my question that IOVA range invalidation is just one
>> aspect of what that notifier might be used for.  The mdev framework
>> also automatically registers and unregisters that notifier around
>> open/release.  So, I don't think we want a new notifier, we just want
>> vfio.c to also consume that notifier.
> 
> Unfortunately the kvm:group attaching happens before device opening,
> so registering the notifier in open() is not functional: the event
> has disappeared before we start watching it.
> 
> A possible workaround is, register the notifier in create() instead of
> open(). That should be functional, but will cause another issue: being able
> to register a notifier means we have a vfio-group reference, when to put
> that reference? putting it in remove() is not a good idea since a device
> might be open/release multiple times between create/remove, holding the ref
> until removal breaks it; putting it in release() is obviously not a
> good idea neither.
> 
> IOW, having the notifiers there must be some dirty work in vendor
> driver to work around the issue above :(
> 
>> So I think this patch needs a few components that build on what Kirti
>> has, 1) we add a blocking_notifier_head per vfio_group and have
>> vfio_{un}regsiter_notifier add and remove that notifier to the group
>> chain, 2) we create a vfio_group_notify() function that the kvm-vfio
>> pseudo device can call via symbol_get, 3) Have kvm-vfio call
>> vfio_group_notify() with VFIO_GROUP_NOTIFY_SET_KVM where the data is a
>> pointer to the struct kvm (or NULL to unset, we don't need separate set
>> vs unset notifiers).  Does that work?  Thanks,
> 
> Yes, it works better than the original form of below patch.
> vfio side doesn't store any data, nor introduce any lock, only a callback
> for kvm to use.
> 

To make my reply clearer: the notifier can work without two separate
set/unset, can be combined with Kirti's iommu notifier, however, the problem
of being too late to register from open() still exists, and I still find it
difficult to work around.

--
Thanks,
Jike