The arm64 port of the Linux kernel provides infrastructure to supportemulation of instructions which have been deprecated, or obsoleted inthe architecture. The infrastructure code uses undefined instructionhooks to support emulation. Where available it also allows turning onthe instruction execution in hardware.The emulation mode can be controlled by writing to sysctl nodes(/proc/sys/abi). The following explains the different executionbehaviours and the corresponding values of the sysctl nodes -* Undef Value: 0 Generates undefined instruction abort. Default for instructions that have been obsoleted in the architecture, e.g., SWP* Emulate Value: 1 Uses software emulation. To aid migration of software, in this mode usage of emulated instruction is traced as well as rate limited warnings are issued. This is the default for deprecated instructions, .e.g., CP15 barriers* Hardware Execution Value: 2 Although marked as deprecated, some implementations may support the enabling/disabling of hardware support for the execution of these instructions. Using hardware execution generally provides better performance, but at the loss of ability to gather runtime statistics about the use of the deprecated instructions.The default mode depends on the status of the instruction in thearchitecture. Deprecated instructions should default to emulationwhile obsolete instructions must be undefined by default.