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diff --git a/Documentation/s390/vfio-ap-locking.rst b/Documentation/s390/vfio-ap-locking.rst new file mode 100644 index 000000000..0dfcdb562 --- /dev/null +++ b/Documentation/s390/vfio-ap-locking.rst @@ -0,0 +1,115 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====================== +VFIO AP Locks Overview +====================== +This document describes the locks that are pertinent to the secure operation +of the vfio_ap device driver. Throughout this document, the following variables +will be used to denote instances of the structures herein described: + +.. code-block:: c + + struct ap_matrix_dev *matrix_dev; + struct ap_matrix_mdev *matrix_mdev; + struct kvm *kvm; + +The Matrix Devices Lock (drivers/s390/crypto/vfio_ap_private.h) +--------------------------------------------------------------- + +.. code-block:: c + + struct ap_matrix_dev { + ... + struct list_head mdev_list; + struct mutex mdevs_lock; + ... + } + +The Matrix Devices Lock (matrix_dev->mdevs_lock) is implemented as a global +mutex contained within the single object of struct ap_matrix_dev. This lock +controls access to all fields contained within each matrix_mdev +(matrix_dev->mdev_list). This lock must be held while reading from, writing to +or using the data from a field contained within a matrix_mdev instance +representing one of the vfio_ap device driver's mediated devices. + +The KVM Lock (include/linux/kvm_host.h) +--------------------------------------- + +.. code-block:: c + + struct kvm { + ... + struct mutex lock; + ... + } + +The KVM Lock (kvm->lock) controls access to the state data for a KVM guest. This +lock must be held by the vfio_ap device driver while one or more AP adapters, +domains or control domains are being plugged into or unplugged from the guest. + +The KVM pointer is stored in the in the matrix_mdev instance +(matrix_mdev->kvm = kvm) containing the state of the mediated device that has +been attached to the KVM guest. + +The Guests Lock (drivers/s390/crypto/vfio_ap_private.h) +----------------------------------------------------------- + +.. code-block:: c + + struct ap_matrix_dev { + ... + struct list_head mdev_list; + struct mutex guests_lock; + ... + } + +The Guests Lock (matrix_dev->guests_lock) controls access to the +matrix_mdev instances (matrix_dev->mdev_list) that represent mediated devices +that hold the state for the mediated devices that have been attached to a +KVM guest. This lock must be held: + +1. 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. + +2. To add matrix_mdev instances to or remove them from matrix_dev->mdev_list. + This is necessary to ensure the proper locking order when the list is perused + to find an ap_matrix_mdev instance for the purpose of plugging/unplugging + AP devices passed through to a KVM guest. + + For example, when a queue device is removed from the vfio_ap device driver, + if the adapter is passed through to a KVM guest, it will have to be + unplugged. In order to figure out whether the adapter is passed through, + the matrix_mdev object to which the queue is assigned will have to be + found. The KVM pointer (matrix_mdev->kvm) can then be used to determine if + the mediated device is passed through (matrix_mdev->kvm != NULL) and if so, + to unplug the adapter. + +It is not necessary to take the Guests Lock to access the KVM pointer if the +pointer is not used to plug/unplug devices passed through to the KVM guest; +however, in this case, the Matrix Devices Lock (matrix_dev->mdevs_lock) must be +held in order to access the KVM pointer since it is set and cleared under the +protection of the Matrix Devices Lock. A case in point is the function that +handles interception of the PQAP(AQIC) instruction sub-function. This handler +needs to access the KVM pointer only for the purposes of setting or clearing IRQ +resources, so only the matrix_dev->mdevs_lock needs to be held. + +The PQAP Hook Lock (arch/s390/include/asm/kvm_host.h) +----------------------------------------------------- + +.. code-block:: c + + typedef int (*crypto_hook)(struct kvm_vcpu *vcpu); + + struct kvm_s390_crypto { + ... + struct rw_semaphore pqap_hook_rwsem; + crypto_hook *pqap_hook; + ... + }; + +The PQAP Hook Lock is a r/w semaphore that controls access to the function +pointer of the handler ``(*kvm->arch.crypto.pqap_hook)`` to invoke when the +PQAP(AQIC) instruction sub-function is intercepted by the host. The lock must be +held in write mode when pqap_hook value is set, and in read mode when the +pqap_hook function is called. |