Re: [PATCH] s390/vfio-ap: do not open code locks for VFIO_GROUP_NOTIFY_SET_KVM notification

Halil Pasic
2021-07-15T13:45:03Z

On Wed, 7 Jul 2021 11:41:56 -0400
Tony Krowiak <akrowiak@linux.ibm.com> wrote:

First sorry for being this late with having a more serious look at the
code.

> @@ -270,6 +270,9 @@ static struct ap_queue_status vfio_ap_irq_enable(struct vfio_ap_queue *q,
> * We take the matrix_dev lock to ensure serialization on queues and
> * mediated device access.
> *
> + * Note: This function must be called with a read lock held on
> + * vcpu->kvm->arch.crypto.pqap_hook_rwsem.
> + *

That is a fine synchronization for the pqap_hook, but I don't think it
is sufficient for everything.

> * Return 0 if we could handle the request inside KVM.
> * otherwise, returns -EOPNOTSUPP to let QEMU handle the fault.
> */
> @@ -287,22 +290,12 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
> return -EOPNOTSUPP;
>
> apqn = vcpu->run->s.regs.gprs[0] & 0xffff;
> - mutex_lock(&matrix_dev->lock);

Here you drop a matrix_dev->lock critical section. And then
you do all the interesting stuff. E.g.
q = vfio_ap_get_queue(matrix_mdev, apqn);
and
vfio_ap_irq_enable(q, status & 0x07, vcpu->run->s.regs.gprs[2]);.
Since in vfio_ap_get_queue() we do the check if the queue belongs
to the given guest, and examine the matrix (apm, aqm) I suppose
that needs to be done holding a lock that protects the matrix,
and to my best knowledge this is still matrix_dev->lock. It would
probably make sense to convert matrix_dev->lock into an rw_semaphore,
or to introduce a some new rwlock which protects less state in the
future, but right now AFAICT it is still matrix_dev->lock.

So I don't think this patch should pass review.

Regards,
Halil

>
> if (!vcpu->kvm->arch.crypto.pqap_hook)
> goto out_unlock;
> matrix_mdev = container_of(vcpu->kvm->arch.crypto.pqap_hook,
> struct ap_matrix_mdev, pqap_hook);
>
> - /*
> - * If the KVM pointer is in the process of being set, wait until the
> - * process has completed.
> - */
> - wait_event_cmd(matrix_mdev->wait_for_kvm,
> - !matrix_mdev->kvm_busy,
> - mutex_unlock(&matrix_dev->lock),
> - mutex_lock(&matrix_dev->lock));
> -
> /* If the there is no guest using the mdev, there is nothing to do */
> if (!matrix_mdev->kvm)
> goto out_unlock;