s390/vfio-ap: introduce new mutex to control access to the KVM pointer

Browse source

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 0cc00c8d40 ("Fix circular lockdep when setting/clearing
  crypto masks")

* see commit 86956e7076 ("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>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

...

This commit is contained in:

Tony Krowiak 2021-09-23 22:21:58 -04:00 • committed by Alexander Gordeev

parent d0786556ca

commit 21195eb038

2 changed files with 7 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

1

drivers/s390/crypto/vfio_ap_drv.c

View file

6

drivers/s390/crypto/vfio_ap_private.h

View file