1===================
2Legacy instructions
3===================
4
5The arm64 port of the Linux kernel provides infrastructure to support
6emulation of instructions which have been deprecated, or obsoleted in
7the architecture. The infrastructure code uses undefined instruction
8hooks to support emulation. Where available it also allows turning on
9the instruction execution in hardware.
10
11The emulation mode can be controlled by writing to sysctl nodes
12(/proc/sys/abi). The following explains the different execution
13behaviours 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
38The default mode depends on the status of the instruction in the
39architecture. Deprecated instructions should default to emulation
40while obsolete instructions must be undefined by default.
41
42Note: Instruction emulation may not be possible in all cases. See
43individual instruction notes for further information.
44
45Supported 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50:Status: Obsolete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.
69