vfio-ap-locking.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3======================
  4VFIO AP Locks Overview
  5======================
  6This document describes the locks that are pertinent to the secure operation
  7of the vfio_ap device driver. Throughout this document, the following variables
  8will be used to denote instances of the structures herein described:
  9
 10.. code-block:: c
 11
 12  struct ap_matrix_dev *matrix_dev;
 13  struct ap_matrix_mdev *matrix_mdev;
 14  struct kvm *kvm;
 15
 16The Matrix Devices Lock (drivers/s390/crypto/vfio_ap_private.h)
 17---------------------------------------------------------------
 18
 19.. code-block:: c
 20
 21  struct ap_matrix_dev {
 22  	...
 23  	struct list_head mdev_list;
 24  	struct mutex mdevs_lock;
 25  	...
 26  }
 27
 28The Matrix Devices Lock (matrix_dev->mdevs_lock) is implemented as a global
 29mutex contained within the single object of struct ap_matrix_dev. This lock
 30controls access to all fields contained within each matrix_mdev
 31(matrix_dev->mdev_list). This lock must be held while reading from, writing to
 32or using the data from a field contained within a matrix_mdev instance
 33representing one of the vfio_ap device driver's mediated devices.
 34
 35The KVM Lock (include/linux/kvm_host.h)
 36---------------------------------------
 37
 38.. code-block:: c
 39
 40  struct kvm {
 41  	...
 42  	struct mutex lock;
 43  	...
 44  }
 45
 46The KVM Lock (kvm->lock) controls access to the state data for a KVM guest. This
 47lock must be held by the vfio_ap device driver while one or more AP adapters,
 48domains or control domains are being plugged into or unplugged from the guest.
 49
 50The KVM pointer is stored in the in the matrix_mdev instance
 51(matrix_mdev->kvm = kvm) containing the state of the mediated device that has
 52been attached to the KVM guest.
 53
 54The Guests Lock (drivers/s390/crypto/vfio_ap_private.h)
 55-----------------------------------------------------------
 56
 57.. code-block:: c
 58
 59  struct ap_matrix_dev {
 60  	...
 61  	struct list_head mdev_list;
 62  	struct mutex guests_lock;
 63  	...
 64  }
 65
 66The Guests Lock (matrix_dev->guests_lock) controls access to the
 67matrix_mdev instances (matrix_dev->mdev_list) that represent mediated devices
 68that hold the state for the mediated devices that have been attached to a
 69KVM guest. This lock must be held:
 70
 711. To control access to the KVM pointer (matrix_mdev->kvm) while the vfio_ap
 72   device driver is using it to plug/unplug AP devices passed through to the KVM
 73   guest.
 74
 752. To add matrix_mdev instances to or remove them from matrix_dev->mdev_list.
 76   This is necessary to ensure the proper locking order when the list is perused
 77   to find an ap_matrix_mdev instance for the purpose of plugging/unplugging
 78   AP devices passed through to a KVM guest.
 79
 80   For example, when a queue device is removed from the vfio_ap device driver,
 81   if the adapter is passed through to a KVM guest, it will have to be
 82   unplugged. In order to figure out whether the adapter is passed through,
 83   the matrix_mdev object to which the queue is assigned will have to be
 84   found. The KVM pointer (matrix_mdev->kvm) can then be used to determine if
 85   the mediated device is passed through (matrix_mdev->kvm != NULL) and if so,
 86   to unplug the adapter.
 87
 88It is not necessary to take the Guests Lock to access the KVM pointer if the
 89pointer is not used to plug/unplug devices passed through to the KVM guest;
 90however, in this case, the Matrix Devices Lock (matrix_dev->mdevs_lock) must be
 91held in order to access the KVM pointer since it is set and cleared under the
 92protection of the Matrix Devices Lock. A case in point is the function that
 93handles interception of the PQAP(AQIC) instruction sub-function. This handler
 94needs to access the KVM pointer only for the purposes of setting or clearing IRQ
 95resources, so only the matrix_dev->mdevs_lock needs to be held.
 96
 97The PQAP Hook Lock (arch/s390/include/asm/kvm_host.h)
 98-----------------------------------------------------
 99
100.. code-block:: c
101
102  typedef int (*crypto_hook)(struct kvm_vcpu *vcpu);
103
104  struct kvm_s390_crypto {
105  	...
106  	struct rw_semaphore pqap_hook_rwsem;
107  	crypto_hook *pqap_hook;
108  	...
109  };
110
111The PQAP Hook Lock is a r/w semaphore that controls access to the function
112pointer of the handler ``(*kvm->arch.crypto.pqap_hook)`` to invoke when the
113PQAP(AQIC) instruction sub-function is intercepted by the host. The lock must be
114held in write mode when pqap_hook value is set, and in read mode when the
115pqap_hook function is called.