1 .. SPDX-License-Identifier: GPL-2.0
2
3 ======================
4 VFIO AP Locks Overview
5 ======================
6 This document describes the locks that are pertinent to the secure operation
7 of the vfio_ap device driver. Throughout this document, the following variables
8 will 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
16 The 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
28 The Matrix Devices Lock (matrix_dev->mdevs_lock) is implemented as a global
29 mutex contained within the single object of struct ap_matrix_dev. This lock
30 controls access to all fields contained within each matrix_mdev
31 (matrix_dev->mdev_list). This lock must be held while reading from, writing to
32 or using the data from a field contained within a matrix_mdev instance
33 representing one of the vfio_ap device driver's mediated devices.
34
35 The 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
46 The KVM Lock (kvm->lock) controls access to the state data for a KVM guest. This
47 lock must be held by the vfio_ap device driver while one or more AP adapters,
48 domains or control domains are being plugged into or unplugged from the guest.
49
50 The 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
52 been attached to the KVM guest.
53
54 The 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
66 The Guests Lock (matrix_dev->guests_lock) controls access to the
67 matrix_mdev instances (matrix_dev->mdev_list) that represent mediated devices
68 that hold the state for the mediated devices that have been attached to a
69 KVM guest. This lock must be held:
70
71 1. 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
75 2. 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
88 It is not necessary to take the Guests Lock to access the KVM pointer if the
89 pointer is not used to plug/unplug devices passed through to the KVM guest;
90 however, in this case, the Matrix Devices Lock (matrix_dev->mdevs_lock) must be
91 held in order to access the KVM pointer since it is set and cleared under the
92 protection of the Matrix Devices Lock. A case in point is the function that
93 handles interception of the PQAP(AQIC) instruction sub-function. This handler
94 needs to access the KVM pointer only for the purposes of setting or clearing IRQ
95 resources, so only the matrix_dev->mdevs_lock needs to be held.
96
97 The 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
111 The PQAP Hook Lock is a r/w semaphore that controls access to the function
112 pointer of the handler ``(*kvm->arch.crypto.pqap_hook)`` to invoke when the
113 PQAP(AQIC) instruction sub-function is intercepted by the host. The lock must be
114 held in write mode when pqap_hook value is set, and in read mode when the
115 pqap_hook function is called.
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