1 .. SPDX-License-Identifier: GPL-2.0-or-later
2
3 ==========================================
4 DEXCR (Dynamic Execution Control Register)
5 ==========================================
6
7 Overview
8 ========
9
10 The DEXCR is a privileged special purpose regi
11 PowerPC ISA 3.1B (Power10) that allows per-cpu
12 execution behaviours. These behaviours include
13 branch target prediction) and enabling return-
14 protection instructions.
15
16 The execution control is exposed in hardware a
17 the DEXCR. Each aspect controls a certain beha
18 to enable/disable the aspect. There are severa
19 different purposes:
20
21 DEXCR
22 A privileged SPR that can control aspects
23 HDEXCR
24 A hypervisor-privileged SPR that can contr
25 enforce aspects for the kernel and userspa
26 UDEXCR
27 An optional ultravisor-privileged SPR that
28
29 Userspace can examine the current DEXCR state
30 provides a non-privileged read-only view of th
31 There is also an SPR that provides a read-only
32 aspects, which ORed with the userspace DEXCR v
33 state for a process.
34
35
36 Configuration
37 =============
38
39 prctl
40 -----
41
42 A process can control its own userspace DEXCR
43 ``PR_PPC_GET_DEXCR`` and ``PR_PPC_SET_DEXCR``
44 :manpage:`prctl(2)` commands. These calls have
45
46 prctl(PR_PPC_GET_DEXCR, unsigned long whic
47 prctl(PR_PPC_SET_DEXCR, unsigned long whic
48
49 The possible 'which' and 'ctrl' values are as
50 between the 'which' value and the DEXCR aspect
51
52 .. flat-table::
53 :header-rows: 1
54 :widths: 2 7 1
55
56 * - ``prctl()`` which
57 - Aspect name
58 - Aspect index
59
60 * - ``PR_PPC_DEXCR_SBHE``
61 - Speculative Branch Hint Enable (SBHE)
62 - 0
63
64 * - ``PR_PPC_DEXCR_IBRTPD``
65 - Indirect Branch Recurrent Target Predic
66 - 3
67
68 * - ``PR_PPC_DEXCR_SRAPD``
69 - Subroutine Return Address Prediction Di
70 - 4
71
72 * - ``PR_PPC_DEXCR_NPHIE``
73 - Non-Privileged Hash Instruction Enable
74 - 5
75
76 .. flat-table::
77 :header-rows: 1
78 :widths: 2 8
79
80 * - ``prctl()`` ctrl
81 - Meaning
82
83 * - ``PR_PPC_DEXCR_CTRL_EDITABLE``
84 - This aspect can be configured with PR_P
85
86 * - ``PR_PPC_DEXCR_CTRL_SET``
87 - This aspect is set / set this aspect
88
89 * - ``PR_PPC_DEXCR_CTRL_CLEAR``
90 - This aspect is clear / clear this aspec
91
92 * - ``PR_PPC_DEXCR_CTRL_SET_ONEXEC``
93 - This aspect will be set after exec / se
94
95 * - ``PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC``
96 - This aspect will be clear after exec /
97
98 Note that
99
100 * which is a plain value, not a bitmask. Aspec
101
102 * ctrl is a bitmask. ``PR_PPC_GET_DEXCR`` retu
103 configuration. For example, ``PR_PPC_GET_DEX
104 ``PR_PPC_DEXCR_CTRL_EDITABLE | PR_PPC_DEXCR_
105 PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC``. This would
106 set, it will be cleared when you run exec, a
107 ``PR_PPC_SET_DEXCR`` prctl.
108
109 * The set/clear terminology refers to setting/
110 For example::
111
112 prctl(PR_PPC_SET_DEXCR, PR_PPC_DEXCR_IBR
113
114 will set the IBRTPD aspect bit in the DEXCR,
115 to be disabled.
116
117 * The status returned by ``PR_PPC_GET_DEXCR``
118 would like applied. It does not include any
119 the hypervisor is enforcing the aspect be se
120 software should read the appropriate SPRs di
121
122 * The aspect state when starting a process is
123 :manpage:`fork(2)`. The state is reset to a
124 :manpage:`execve(2)`. The PR_PPC_SET_DEXCR p
125 values.
126
127 * The ``*_ONEXEC`` controls do not change the
128
129 Use ``PR_PPC_SET_DEXCR`` with one of ``PR_PPC_
130 ``PR_PPC_DEXCR_CTRL_CLEAR`` to edit a given as
131
132 Common error codes for both getting and settin
133
134 .. flat-table::
135 :header-rows: 1
136 :widths: 2 8
137
138 * - Error
139 - Meaning
140
141 * - ``EINVAL``
142 - The DEXCR is not supported by the kerne
143
144 * - ``ENODEV``
145 - The aspect is not recognised by the ker
146 hardware.
147
148 ``PR_PPC_SET_DEXCR`` may also report the follo
149
150 .. flat-table::
151 :header-rows: 1
152 :widths: 2 8
153
154 * - Error
155 - Meaning
156
157 * - ``EINVAL``
158 - The ctrl value contains unrecognised fl
159
160 * - ``EINVAL``
161 - The ctrl value contains mutually confli
162 ``PR_PPC_DEXCR_CTRL_SET | PR_PPC_DEXCR_
163
164 * - ``EPERM``
165 - This aspect cannot be modified with prc
166 PR_PPC_DEXCR_CTRL_EDITABLE flag with PR
167
168 * - ``EPERM``
169 - The process does not have sufficient pr
170 For example, clearing NPHIE on exec is
171 can still clear its own NPHIE aspect wi
172
173 This interface allows a process to control its
174 the initial DEXCR value for any children in it
175 child to use an ``*_ONEXEC`` control). This al
176 default value of the DEXCR, for example allowi
177 default values.
178
179
180 coredump and ptrace
181 ===================
182
183 The userspace values of the DEXCR and HDEXCR (
184 ``NT_PPC_DEXCR``. These are each 64 bits and r
185 assist with core dumps. The DEXCR may be made
186 bits of both registers (corresponding to the n
187
188 If the kernel config ``CONFIG_CHECKPOINT_RESTO
189 ``NT_PPC_HASHKEYR`` is available and exposes t
190 for reading and writing. This is a tradeoff be
191 checkpoint/restore support: a process should n
192 secret key, but restoring a process requires s
193 therefore appears in core dumps, and an attack
194 a coredump and effectively bypass ROP protecti
195 key (potentially all threads from the same par
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