[PATCH v20 11/20] s390/vfio-ap: prepare for dynamic update of guest's APCB on queue probe/remove

Tony Krowiak
2022-06-21T15:52:24Z

The callback functions for probing and removing a queue device 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

A new helper function is introduced to be used by the probe callback to
acquire the required locks. Since the probe callback only has
access to a queue device when it is called, the helper function will find
the ap_matrix_mdev object to which the queue device's APQN is assigned and
return it so the KVM guest to which the mdev is attached can be dynamically
updated.

Note that in order to find the ap_matrix_mdev (matrix_mdev) object, it is
necessary to search the matrix_dev->mdev_list. This presents a
locking order dilemma because the matrix_dev->mdevs_lock can't be taken to
protect against changes to the list while searching for the matrix_mdev to
which a queue device's APQN is assigned. This is due to the fact that the
proper locking order requires that the matrix_dev->mdevs_lock be taken
after both the matrix_mdev->kvm->lock and the matrix_dev->mdevs_lock.
Consequently, the matrix_dev->guests_lock will be used to protect against
removal of a matrix_mdev object from the list while a queue device is
being probed. This necessitates changes to the mdev probe/remove
callback functions to take the matrix_dev->guests_lock prior to removing
a matrix_mdev object from the list.

A new macro is also introduced to acquire the locks required to dynamically
update the guest's APCB in the proper order when a queue device is
removed.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
---
drivers/s390/crypto/vfio_ap_ops.c | 123 ++++++++++++++++++++----------
1 file changed, 84 insertions(+), 39 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index b8f901e6b580..6d55713c7f7a 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -120,6 +120,71 @@ static const struct vfio_device_ops vfio_ap_matrix_dev_ops;
mutex_unlock(&matrix_dev->guests_lock); \
})

+/**
+ * get_update_locks_by_apqn: Find the mdev to which an APQN is assigned and
+ * acquire the locks required to update the APCB of
+ * the KVM guest to which the mdev is attached.
+ *
+ * @apqn: the APQN of a queue device.
+ *
+ * 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 @apqn is not assigned to a matrix_mdev, the matrix_mdev->kvm->lock
+ * will not be taken.
+ *
+ * Return: the ap_matrix_mdev object to which @apqn is assigned or NULL if @apqn
+ * is not assigned to an ap_matrix_mdev.
+ */
+static struct ap_matrix_mdev *get_update_locks_by_apqn(int apqn)
+{
+ struct ap_matrix_mdev *matrix_mdev;
+
+ mutex_lock(&matrix_dev->guests_lock);
+
+ list_for_each_entry(matrix_mdev, &matrix_dev->mdev_list, node) {
+ if (test_bit_inv(AP_QID_CARD(apqn), matrix_mdev->matrix.apm) &&
+ test_bit_inv(AP_QID_QUEUE(apqn), matrix_mdev->matrix.aqm)) {
+ if (matrix_mdev->kvm)
+ mutex_lock(&matrix_mdev->kvm->lock);
+
+ mutex_lock(&matrix_dev->mdevs_lock);
+
+ return matrix_mdev;
+ }
+ }
+
+ mutex_lock(&matrix_dev->mdevs_lock);
+
+ return NULL;
+}
+
+/**
+ * get_update_locks_for_queue: get the locks required to update the APCB of the
+ * KVM guest to which the matrix mdev linked to a
+ * vfio_ap_queue object is attached.
+ *
+ * @q: a pointer to a vfio_ap_queue object.
+ *
+ * The proper locking order is:
+ * 1. q->matrix_dev->guests_lock: required to use the KVM pointer to update a
+ * KVM guest's APCB.
+ * 2. q->matrix_mdev->kvm->lock: required to update a guest's APCB
+ * 3. matrix_dev->mdevs_lock: required to access data stored in matrix_mdev
+ *
+ * Note: if @queue is not linked to an ap_matrix_mdev object, the KVM lock
+ * will not be taken.
+ */
+#define get_update_locks_for_queue(q) ({ \
+ mutex_lock(&matrix_dev->guests_lock); \
+ if (q->matrix_mdev && q->matrix_mdev->kvm) \
+ mutex_lock(&q->matrix_mdev->kvm->lock); \
+ mutex_lock(&matrix_dev->mdevs_lock); \
+})
+
/**
* vfio_ap_mdev_get_queue - retrieve a queue with a specific APQN from a
* hash table of queues assigned to a matrix mdev
@@ -618,21 +683,17 @@ static int vfio_ap_mdev_probe(struct mdev_device *mdev)
matrix_mdev->pqap_hook = handle_pqap;
vfio_ap_matrix_init(&matrix_dev->info, &matrix_mdev->shadow_apcb);
hash_init(matrix_mdev->qtable.queues);
- dev_set_drvdata(&mdev->dev, matrix_mdev);
- mutex_lock(&matrix_dev->mdevs_lock);
- list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
- mutex_unlock(&matrix_dev->mdevs_lock);

ret = vfio_register_emulated_iommu_dev(&matrix_mdev->vdev);
if (ret)
goto err_list;
dev_set_drvdata(&mdev->dev, matrix_mdev);
+ mutex_lock(&matrix_dev->mdevs_lock);
+ list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
+ mutex_unlock(&matrix_dev->mdevs_lock);
return 0;

err_list:
- mutex_lock(&matrix_dev->mdevs_lock);
- list_del(&matrix_mdev->node);
- mutex_unlock(&matrix_dev->mdevs_lock);
vfio_uninit_group_dev(&matrix_mdev->vdev);
kfree(matrix_mdev);
err_dec_available:
@@ -695,11 +756,13 @@ static void vfio_ap_mdev_remove(struct mdev_device *mdev)

vfio_unregister_group_dev(&matrix_mdev->vdev);

+ mutex_lock(&matrix_dev->guests_lock);
mutex_lock(&matrix_dev->mdevs_lock);
vfio_ap_mdev_reset_queues(matrix_mdev);
vfio_ap_mdev_unlink_fr_queues(matrix_mdev);
list_del(&matrix_mdev->node);
mutex_unlock(&matrix_dev->mdevs_lock);
+ mutex_unlock(&matrix_dev->guests_lock);
vfio_uninit_group_dev(&matrix_mdev->vdev);
kfree(matrix_mdev);
atomic_inc(&matrix_dev->available_instances);
@@ -1697,32 +1760,11 @@ void vfio_ap_mdev_unregister(void)
mdev_unregister_driver(&vfio_ap_matrix_driver);
}

-/*
- * vfio_ap_queue_link_mdev
- *
- * @q: The queue to link with the matrix mdev.
- *
- * Links @q with the matrix mdev to which the queue's APQN is assigned.
- */
-static void vfio_ap_queue_link_mdev(struct vfio_ap_queue *q)
-{
- unsigned long apid = AP_QID_CARD(q->apqn);
- unsigned long apqi = AP_QID_QUEUE(q->apqn);
- struct ap_matrix_mdev *matrix_mdev;
-
- list_for_each_entry(matrix_mdev, &matrix_dev->mdev_list, node) {
- if (test_bit_inv(apid, matrix_mdev->matrix.apm) &&
- test_bit_inv(apqi, matrix_mdev->matrix.aqm)) {
- vfio_ap_mdev_link_queue(matrix_mdev, q);
- break;
- }
- }
-}
-
int vfio_ap_mdev_probe_queue(struct ap_device *apdev)
{
int ret;
struct vfio_ap_queue *q;
+ struct ap_matrix_mdev *matrix_mdev;

ret = sysfs_create_group(&apdev->device.kobj, &vfio_queue_attr_group);
if (ret)
@@ -1732,17 +1774,18 @@ int vfio_ap_mdev_probe_queue(struct ap_device *apdev)
if (!q)
return -ENOMEM;

- mutex_lock(&matrix_dev->mdevs_lock);
q->apqn = to_ap_queue(&apdev->device)->qid;
q->saved_isc = VFIO_AP_ISC_INVALID;
- vfio_ap_queue_link_mdev(q);
- if (q->matrix_mdev) {
- vfio_ap_mdev_filter_matrix(q->matrix_mdev->matrix.apm,
- q->matrix_mdev->matrix.aqm,
- q->matrix_mdev);
+ matrix_mdev = get_update_locks_by_apqn(q->apqn);
+
+ if (matrix_mdev) {
+ vfio_ap_mdev_link_queue(matrix_mdev, q);
+ vfio_ap_mdev_filter_matrix(matrix_mdev->matrix.apm,
+ matrix_mdev->matrix.aqm,
+ matrix_mdev);
}
dev_set_drvdata(&apdev->device, q);
- mutex_unlock(&matrix_dev->mdevs_lock);
+ release_update_locks_for_mdev(matrix_mdev);

return 0;
}
@@ -1751,12 +1794,14 @@ void vfio_ap_mdev_remove_queue(struct ap_device *apdev)
{
unsigned long apid;
struct vfio_ap_queue *q;
+ struct ap_matrix_mdev *matrix_mdev;

- mutex_lock(&matrix_dev->mdevs_lock);
sysfs_remove_group(&apdev->device.kobj, &vfio_queue_attr_group);
q = dev_get_drvdata(&apdev->device);
+ get_update_locks_for_queue(q);
+ matrix_mdev = q->matrix_mdev;

- if (q->matrix_mdev) {
+ if (matrix_mdev) {
vfio_ap_unlink_queue_fr_mdev(q);

apid = AP_QID_CARD(q->apqn);
@@ -1767,5 +1812,5 @@ void vfio_ap_mdev_remove_queue(struct ap_device *apdev)
vfio_ap_mdev_reset_queue(q, 1);
dev_set_drvdata(&apdev->device, NULL);
kfree(q);
- mutex_unlock(&matrix_dev->mdevs_lock);
+ release_update_locks_for_mdev(matrix_mdev);
}
--
2.35.3