linux-kernel - Re: [PATCH] s390/vfio-ap: do not open code locks for VFIO_GROUP_NOTIFY_SET

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Date:   Thu, 15 Jul 2021 15:44:51 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        frankja@...ux.ibm.com, david@...hat.com, imbrenda@...ux.ibm.com,
        hca@...ux.ibm.com
Subject: Re: [PATCH] s390/vfio-ap: do not open code locks for
 VFIO_GROUP_NOTIFY_SET_KVM notification

On Wed,  7 Jul 2021 11:41:56 -0400
Tony Krowiak <akrowiak@...ux.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;