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