Re: [PATCH v6 3/7] s390: ap: setup relation betwen KVM and mediated device

[Tony Krowiak <akrowiak@xxxxxxxxxxxxx>]

On 3/29/19 4:58 AM, Pierre Morel wrote:
> On 28/03/2019 18:25, Tony Krowiak wrote:
> > On 3/28/19 12:27 PM, Pierre Morel wrote:
> > > On 28/03/2019 17:12, Tony Krowiak wrote:
> > > > On 3/22/19 10:43 AM, Pierre Morel wrote:
> > > > > When the mediated device is open we setup the relation with KVM 
> > > > > unset it
> > > > > when the mediated device is released.
> > > >
> > > > s/open we setup/open, we set up/
> > > > s/with KVM unset/with KVM and unset/
> > > >
> > > > > We lock the matrix mediated device to avoid any change until the
> > > > > open is done.
> > > > > We make sure that KVM is present when opening the mediated device
> > > > > otherwise we return an error.
> > > >
> > > > s/mediated device/mediated device,/
> > > >
> > > > > Increase kvm's refcount to ensure the KVM structures are still 
> > > > > available
> > > > > during the use of the mediated device by the guest.
> > > > >
> > > > > Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> > > > > ---
> > > > >   drivers/s390/crypto/vfio_ap_ops.c | 143 
> > > > > +++++++++++++++++++++-----------------
> > > > >   1 file changed, 79 insertions(+), 64 deletions(-)
> > > > >
> > > > > diff --git a/drivers/s390/crypto/vfio_ap_ops.c 
> > > > > b/drivers/s390/crypto/vfio_ap_ops.c
> > > > > index 77f7bac..bdb36e0 100644
> > > > > --- a/drivers/s390/crypto/vfio_ap_ops.c
> > > > > +++ b/drivers/s390/crypto/vfio_ap_ops.c
> > > > > @@ -787,74 +787,24 @@ static const struct attribute_group 
> > > > > *vfio_ap_mdev_attr_groups[] = {
> > > > >       NULL
> > > > >   };
> > > > > -/**
> > > > > - * vfio_ap_mdev_set_kvm
> > > > > - *
> > > > > - * @matrix_mdev: a mediated matrix device
> > > > > - * @kvm: reference to KVM instance
> > > > > - *
> > > > > - * Verifies no other mediated matrix device has @kvm and sets a 
> > > > > reference to
> > > > > - * it in @matrix_mdev->kvm.
> > > > > - *
> > > > > - * Return 0 if no other mediated matrix device has a reference to 
> > > > > @kvm;
> > > > > - * otherwise, returns an -EPERM.
> > > > > - */
> > > > > -static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
> > > > > -                struct kvm *kvm)
> > > > > -{
> > > > > -    struct ap_matrix_mdev *m;
> > > > > -
> > > > > -    mutex_lock(&matrix_dev->lock);
> > > > > -
> > > > > -    list_for_each_entry(m, &matrix_dev->mdev_list, node) {
> > > > > -        if ((m != matrix_mdev) && (m->kvm == kvm)) {
> > > > > -            mutex_unlock(&matrix_dev->lock);
> > > > > -            return -EPERM;
> > > > > -        }
> > > > > -    }
> > > > > -
> > > > > -    matrix_mdev->kvm = kvm;
> > > > > -    mutex_unlock(&matrix_dev->lock);
> > > > > -
> > > > > -    return 0;
> > > > > -}
> > > > > -
> > > > >   static int vfio_ap_mdev_group_notifier(struct notifier_block *nb,
> > > > >                          unsigned long action, void *data)
> > > > >   {
> > > > > -    int ret;
> > > > >       struct ap_matrix_mdev *matrix_mdev;
> > > > >       if (action != VFIO_GROUP_NOTIFY_SET_KVM)
> > > > >           return NOTIFY_OK;
> > > > >       matrix_mdev = container_of(nb, struct ap_matrix_mdev, 
> > > > > group_notifier);
> > > > > -
> > > > > -    if (!data) {
> > > > > -        matrix_mdev->kvm = NULL;
> > > > > -        return NOTIFY_OK;
> > > > > -    }
> > > > > -
> > > > > -    ret = vfio_ap_mdev_set_kvm(matrix_mdev, data);
> > > > > -    if (ret)
> > > > > -        return NOTIFY_DONE;
> > > > > -
> > > > > -    /* If there is no CRYCB pointer, then we can't copy the masks */
> > > > > -    if (!matrix_mdev->kvm->arch.crypto.crycbd)
> > > > > -        return NOTIFY_DONE;
> > > > > -
> > > > > -    kvm_arch_crypto_set_masks(matrix_mdev->kvm, 
> > > > > matrix_mdev->matrix.apm,
> > > > > -                  matrix_mdev->matrix.aqm,
> > > > > -                  matrix_mdev->matrix.adm);
> > > > > +    matrix_mdev->kvm = data;
> > > > >       return NOTIFY_OK;
> > > > >   }
> > > > > -static int vfio_ap_mdev_reset_queues(struct mdev_device *mdev)
> > > > > +static int vfio_ap_mdev_reset_queues(struct ap_matrix_mdev 
> > > > > *matrix_mdev)
> > > > >   {
> > > > >       int ret;
> > > > >       int rc = 0;
> > > > > -    struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
> > > > >       struct vfio_ap_queue *q;
> > > > >       list_for_each_entry(q, &matrix_mdev->qlist, list) {
> > > > > @@ -871,41 +821,106 @@ static int vfio_ap_mdev_reset_queues(struct 
> > > > > mdev_device *mdev)
> > > > >       return rc;
> > > > >   }
> > > > > +/**
> > > > > + * vfio_ap_mdev_set_kvm
> > > > > + *
> > > > > + * @matrix_mdev: a mediated matrix device
> > > > > + *
> > > > > + * - Verifies that the hook is free and install the PQAP hook
> > > > > + * - Copy the matrix masks inside the CRYCB
> > > > > + * - Increment the KVM rerference count
> > > > > + *
> > > > > + * Return 0 if no other mediated matrix device has a reference to 
> > > > > @kvm;
> > > > > + * otherwise, returns an -EPERM.
> > > > > + */
> > > > > +static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev)
> > > > > +{
> > > > > +    if (matrix_mdev->kvm->arch.crypto.pqap_hook)
> > > > > +        return -EPERM;
> > > >
> > > > How would this happen; in other words, why are we checking this?
> > >
> > > I check this to verify that no other AP mediated device is already in 
> > > use by this VM.
> >
> > Maybe you should insert a comment to that effect.
>
> Please notice that there is already a comment on this in the description 
> of the function.

True, but that comment merely states that the function verifies the
hook is free, not the reason why that particular check is done. When
I reviewed the code and saw this check, I wondered why it was necessary.
The comment you have would not have helped in this regard, so maybe
you need to update your comment.

> Regards,
> Pierre