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SubjectRe: [PATCH v19 08/20] s390/vfio-ap: introduce new mutex to control access to the KVM pointer
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On 4/4/22 18:10, Tony Krowiak wrote:
> The vfio_ap device driver registers for notification when the pointer to
> the KVM object for a guest is set. Recall that the KVM lock (kvm->lock)
> mutex must be taken outside of the matrix_dev->lock mutex to prevent the
> reporting by lockdep of a circular locking dependency (a.k.a., a lockdep
> splat):
>
> * see commit 0cc00c8d4050 ("Fix circular lockdep when setting/clearing
> crypto masks")
>
> * see commit 86956e70761b ("replace open coded locks for
> VFIO_GROUP_NOTIFY_SET_KVM notification")
>
> With the introduction of support for hot plugging/unplugging AP devices
> passed through to a KVM guest, a new guests_lock mutex is introduced to
> ensure the proper locking order is maintained:
>
> struct ap_matrix_dev {
> ...
> struct mutex guests_lock;
> ...
> }
>
> The matrix_dev->guests_lock controls access to the matrix_mdev instances
> that hold the state for AP devices that have been passed through to a
> KVM guest. This lock must be held to control access to the KVM pointer
> (matrix_mdev->kvm) while the vfio_ap device driver is using it to
> plug/unplug AP devices passed through to the KVM guest.
>
> Keep in mind, the proper locking order must be maintained whenever
> dynamically updating a KVM guest's APCB to plug/unplug adapters, domains
> and control domains:
>
> 1. matrix_dev->guests_lock: required to use the KVM pointer - stored in
> a struct ap_matrix_mdev instance - 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
> struct ap_matrix_mdev instance.
>
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> ---
> drivers/s390/crypto/vfio_ap_drv.c | 1 +
> drivers/s390/crypto/vfio_ap_private.h | 6 ++++++
> 2 files changed, 7 insertions(+)
>
> diff --git a/drivers/s390/crypto/vfio_ap_drv.c b/drivers/s390/crypto/vfio_ap_drv.c
> index 0a5acd151a9b..c258e5f7fdfc 100644
> --- a/drivers/s390/crypto/vfio_ap_drv.c
> +++ b/drivers/s390/crypto/vfio_ap_drv.c
> @@ -161,6 +161,7 @@ static int vfio_ap_matrix_dev_create(void)
>
> mutex_init(&matrix_dev->mdevs_lock);
> INIT_LIST_HEAD(&matrix_dev->mdev_list);
> + mutex_init(&matrix_dev->guests_lock);
>
> dev_set_name(&matrix_dev->device, "%s", VFIO_AP_DEV_NAME);
> matrix_dev->device.parent = root_device;
> diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
> index 5262e02192a4..ec926f2f2930 100644
> --- a/drivers/s390/crypto/vfio_ap_private.h
> +++ b/drivers/s390/crypto/vfio_ap_private.h
> @@ -39,6 +39,11 @@
> * single ap_matrix_mdev device. It's quite coarse but we don't
> * expect much contention.
> * @vfio_ap_drv: the vfio_ap device driver
> + * @guests_lock: mutex for controlling access to a guest that is using AP
> + * devices passed through by the vfio_ap device driver. This lock
> + * will be taken when the AP devices are plugged into or unplugged
> + * from a guest, and when an ap_matrix_mdev device is added to or
> + * removed from @mdev_list or the list is iterated.
> */
> struct ap_matrix_dev {
> struct device device;
> @@ -47,6 +52,7 @@ struct ap_matrix_dev {
> struct list_head mdev_list;
> struct mutex mdevs_lock;
> struct ap_driver *vfio_ap_drv;
> + struct mutex guests_lock;
> };
>
> extern struct ap_matrix_dev *matrix_dev;

In isolation... Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>


--
-- Jason J. Herne (jjherne@linux.ibm.com)

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