1 ===================
2 Legacy instructions
3 ===================
4
5 The arm64 port of the Linux kernel provides infrastructure to support
6 emulation of instructions which have been deprecated, or obsoleted in
7 the architecture. The infrastructure code uses undefined instruction
8 hooks to support emulation. Where available it also allows turning on
9 the instruction execution in hardware.
10
11 The emulation mode can be controlled by writing to sysctl nodes
12 (/proc/sys/abi). The following explains the different execution
13 behaviours and the corresponding values of the sysctl nodes -
14
15 * Undef
16 Value: 0
17
18 Generates undefined instruction abort. Default for instructions that
19 have been obsoleted in the architecture, e.g., SWP
20
21 * Emulate
22 Value: 1
23
24 Uses software emulation. To aid migration of software, in this mode
25 usage of emulated instruction is traced as well as rate limited
26 warnings are issued. This is the default for deprecated
27 instructions, .e.g., CP15 barriers
28
29 * Hardware Execution
30 Value: 2
31
32 Although marked as deprecated, some implementations may support the
33 enabling/disabling of hardware support for the execution of these
34 instructions. Using hardware execution generally provides better
35 performance, but at the loss of ability to gather runtime statistics
36 about the use of the deprecated instructions.
37
38 The default mode depends on the status of the instruction in the
39 architecture. Deprecated instructions should default to emulation
40 while obsolete instructions must be undefined by default.
41
42 Note: Instruction emulation may not be possible in all cases. See
43 individual instruction notes for further information.
44
45 Supported legacy instructions
46 -----------------------------
47 * SWP{B}
48
49 :Node: /proc/sys/abi/swp
50 :Status: Obsolete
51 :Default: Undef (0)
52
53 * CP15 Barriers
54
55 :Node: /proc/sys/abi/cp15_barrier
56 :Status: Deprecated
57 :Default: Emulate (1)
58
59 * SETEND
60
61 :Node: /proc/sys/abi/setend
62 :Status: Deprecated
63 :Default: Emulate (1)*
64
65 Note: All the cpus on the system must have mixed endian support at EL0
66 for this feature to be enabled. If a new CPU - which doesn't support mixed
67 endian - is hotplugged in after this feature has been enabled, there could
68 be unexpected results in the application.
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