linux-kernel - Re: [PATCH v19 10/20] s390/vfio-ap: prepare for dynamic update of guest's APCB on assign/unassign

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Message-ID: <67f17a73-28e2-d458-a052-2782e16fe96d@linux.ibm.com>
Date:   Fri, 27 May 2022 09:18:56 -0400
From:   "Jason J. Herne" <jjherne@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     freude@...ux.ibm.com, borntraeger@...ibm.com, cohuck@...hat.com,
        mjrosato@...ux.ibm.com, pasic@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com
Subject: Re: [PATCH v19 10/20] s390/vfio-ap: prepare for dynamic update of
 guest's APCB on assign/unassign

On 4/4/22 18:10, Tony Krowiak wrote:
> The functions backing the matrix mdev's sysfs attribute interfaces to
> assign/unassign adapters, domains and control domains must take and
> release the locks required to perform a dynamic update of a guest's APCB
> in the proper order.
> 
> The proper order for taking the locks is:
> 
> matrix_dev->guests_lock => kvm->lock => matrix_dev->mdevs_lock
> 
> The proper order for releasing the locks is:
> 
> matrix_dev->mdevs_lock => kvm->lock => matrix_dev->guests_lock
> 
> Two new macros are introduced for this purpose: One to take the locks and
> the other to release the locks. These macros will be used by the
> assignment/unassignment functions to prepare for dynamic update of
> the KVM guest's APCB.
> 
> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>
> ---
>   drivers/s390/crypto/vfio_ap_ops.c | 69 +++++++++++++++++++++++++------
>   1 file changed, 57 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 757bbf449b04..2219b1069ceb 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -71,6 +71,51 @@ static const struct vfio_device_ops vfio_ap_matrix_dev_ops;
>   	mutex_unlock(&matrix_dev->guests_lock);	\
>   })
>   
> +/**
> + * get_update_locks_for_mdev: Acquire the locks required to dynamically update a
> + *			      KVM guest's APCB in the proper order.
> + *
> + * @matrix_mdev: a pointer to a struct ap_matrix_mdev object containing the AP
> + *		 configuration data to use to update a KVM guest's APCB.
> + *
> + * The proper locking order is:
> + * 1. matrix_dev->guests_lock: required to use the KVM pointer to update a KVM
> + *			       guest's APCB.
> + * 2. matrix_mdev->kvm->lock:  required to update a guest's APCB
> + * 3. matrix_dev->mdevs_lock:  required to access data stored in a matrix_mdev
> + *
> + * Note: If @matrix_mdev is NULL or is not attached to a KVM guest, the KVM
> + *	 lock will not be taken.
> + */

Perhaps the locking order should be documented once at the top of all of the locking
functions instead of in each comment. The current method seems needlessly verbose.

> +#define get_update_locks_for_mdev(matrix_mdev) ({	\
> +	mutex_lock(&matrix_dev->guests_lock);		\
> +	if (matrix_mdev && matrix_mdev->kvm)		\
> +		mutex_lock(&matrix_mdev->kvm->lock);	\
> +	mutex_lock(&matrix_dev->mdevs_lock);		\
> +})

It does not make sense to reference matrix_dev on the first line of this macro and
then check it for a null value on the next line. If it can be null then the check
needs to come before the usage. If it cannot be null, then we can remove the check.
Same comment for the release macro.

> +/**
> + * release_update_locks_for_mdev: Release the locks used to dynamically update a
> + *				  KVM guest's APCB in the proper order.
> + *
> + * @matrix_mdev: a pointer to a struct ap_matrix_mdev object containing the AP
> + *		 configuration data to use to update a KVM guest's APCB.
> + *
> + * The proper unlocking order is:
> + * 1. matrix_dev->mdevs_lock
> + * 2. matrix_mdev->kvm->lock
> + * 3. matrix_dev->guests_lock
> + *
> + * Note: If @matrix_mdev is NULL or is not attached to a KVM guest, the KVM
> + *	 lock will not be released.
> + */
> +#define release_update_locks_for_mdev(matrix_mdev) ({	\
> +	mutex_unlock(&matrix_dev->mdevs_lock);		\
> +	if (matrix_mdev && matrix_mdev->kvm)		\
> +		mutex_unlock(&matrix_mdev->kvm->lock);		\
> +	mutex_unlock(&matrix_dev->guests_lock);		\
> +})
> +
-- 
-- Jason J. Herne (jjherne@...ux.ibm.com)