Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM pointer invalidated

[Tony Krowiak <akrowiak@linux.ibm.com>]

On 12/7/20 7:40 PM, Halil Pasic wrote:
> On Mon, 7 Dec 2020 14:05:55 -0500
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>>
>> On 12/2/20 6:41 PM, Tony Krowiak wrote:
>>> The vfio_ap device driver registers a group notifier with VFIO when the
>>> file descriptor for a VFIO mediated device for a KVM guest is opened to
>>> receive notification that the KVM pointer is set (VFIO_GROUP_NOTIFY_SET_KVM
>>> event). When the KVM pointer is set, the vfio_ap driver stashes the pointer
>>> and calls the kvm_get_kvm() function to increment its reference counter.
>>> When the notifier is called to make notification that the KVM pointer has
>>> been set to NULL, the driver should clean up any resources associated with
>>> the KVM pointer and decrement its reference counter. The current
>>> implementation does not take care of this clean up.
>>>
>>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>>> ---
>>>    drivers/s390/crypto/vfio_ap_ops.c | 21 +++++++++++++--------
>>>    1 file changed, 13 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>>> index e0bde8518745..eeb9c9130756 100644
>>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>>> @@ -1083,6 +1083,17 @@ static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
>>>    	return NOTIFY_DONE;
>>>    }
>>>    
>>> +static void vfio_ap_mdev_put_kvm(struct ap_matrix_mdev *matrix_mdev)
>>> +{
>>> +	if (matrix_mdev->kvm) {
>>> +		kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
>>> +		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
>>> +		vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
>> This reset probably does not belong here since there is no
>> reason to reset the queues in the group notifier (see below).
> What about kvm_s390_gisc_unregister()? That needs a valid kvm
> pointer, or? Or is it OK to not pair a kvm_s390_gisc_register()
> with an kvm_s390_gisc_unregister()?

I probably should have been more specific about what I meant.
I was thinking that the reset should not be dependent upon
whether there is a KVM pointer or not since this function is
also called from the release callback. On the other hand,
the vfio_ap_mdev_reset_queues function calls the
vfio_ap_irq_disable (AQIC) function after each queue is reset.
The vfio_ap_irq_disable function also cleans up the AQIC
resources which requires that the KVM point is valid, so if
the vfio_ap_reset_queues function is not called with a
valid KVM pointer, that could result in an exception.

The thing is, it is unnecessary to disable interrupts after
resetting a queue because the reset disables interrupts,
so I think I should include a patch for this fix that does the
following:

1. Removes the disabling of interrupts subsequent to resetting
     a queue.
2. Includes the cleanup of AQIC resources when a queue is
     reset if a KVM pointer is present.

This will allow us to keep the reset in the function above as well
as the other places from which reset is executed.

>
> Regards,
> Halil
>
>> The reset should be done in the release callback only regardless
>> of whether the KVM pointer exists or not.
>>
>>> +		kvm_put_kvm(matrix_mdev->kvm);
>>> +		matrix_mdev->kvm = NULL;
>>> +	}
>>> +}
>>> +
>>>    static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
>>>    				       unsigned long action, void *data)
>>>    {
>>> @@ -1095,7 +1106,7 @@ static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
>>>    	matrix_mdev = container_of(nb, struct ap_matrix_mdev, group_notifier);
>>>    
>>>    	if (!data) {
>>> -		matrix_mdev->kvm = NULL;
>>> +		vfio_ap_mdev_put_kvm(matrix_mdev);
>>>    		return NOTIFY_OK;
>>>    	}
>>>    
>>> @@ -1222,13 +1233,7 @@ static void vfio_ap_mdev_release(struct mdev_device *mdev)
>>>    	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>>>    
>>>    	mutex_lock(&matrix_dev->lock);
>>> -	if (matrix_mdev->kvm) {
>>> -		kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
>>> -		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
>>> -		vfio_ap_mdev_reset_queues(mdev);
>> This release should be moved outside of the block and
>> performed regardless of whether the KVM pointer exists or
>> not.
>>
>>> -		kvm_put_kvm(matrix_mdev->kvm);
>>> -		matrix_mdev->kvm = NULL;
>>> -	}
>>> +	vfio_ap_mdev_put_kvm(matrix_mdev);
>>>    	mutex_unlock(&matrix_dev->lock);
>>>    
>>>    	vfio_unregister_notifier(mdev_dev(mdev), VFIO_IOMMU_NOTIFY,