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

[Tony Krowiak]

On 12/3/20 12:55 PM, Halil Pasic wrote:
> On Wed,  2 Dec 2020 18:41:01 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Do we need a Fixes tag? Do we need this backported? In my opinion
> this is necessary since the interrupt patches.

I'll put in a fixes tag:
Fixes: 258287c994de (s390: vfio-ap: implement mediated device open callback)

Yes, this should probably be backported.

> > ---
> >   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)
> I don't like the name. The function does more that put_kvm. Maybe
> something  like _disconnect_kvm()?
Since the vfio_ap_mdev_set_kvm() function is called by the
notifier when the KVM pointer is set, how about:

vfio_ap_mdev_unset_kvm()

for when the KVM pointer is nullified?

> > +{
> > +	if (matrix_mdev->kvm) {
> > +		(matrix_mdev->kvm);
> > +		matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> Is a plain assignment to arch.crypto.pqap_hook apropriate, or do we need
> to take more care?
>
> For instance kvm_arch_crypto_set_masks() takes kvm->lock before poking
> kvm->arch.crypto.crycb.
>
> > +		vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
> > +		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);
> The lock question was already raised.
>
> What are the exact circumstances under which this branch can be taken?
>
> >   		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);
> > -		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,