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44 45Supported legacy instructions 46----------------------------- 47* SWP{B} 48 49:Node: /proc/sys/abi/swp 50:Status: Obsolete 51:Default: Undef (0) 52 53* CP15 Barriers 54 55:Node: /proc/sys/abi/cp15_barrier 56:Status: Deprecated 57:Default: Emulate (1) 58 59* SETEND 60 61:Node: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