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 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.

--
Thanks,
Jike