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| -rw-r--r-- | Documentation/arm64/legacy_instructions.txt | 57 |
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diff --git a/Documentation/arm64/legacy_instructions.txt b/Documentation/arm64/legacy_instructions.txt new file mode 100644 index 000000000..01bf3d9fa --- /dev/null +++ b/Documentation/arm64/legacy_instructions.txt @@ -0,0 +1,57 @@ +The arm64 port of the Linux kernel provides infrastructure to support +emulation of instructions which have been deprecated, or obsoleted in +the architecture. The infrastructure code uses undefined instruction +hooks to support emulation. Where available it also allows turning on +the instruction execution in hardware. + +The emulation mode can be controlled by writing to sysctl nodes +(/proc/sys/abi). The following explains the different execution +behaviours 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 the +architecture. Deprecated instructions should default to emulation +while obsolete instructions must be undefined by default. + +Note: Instruction emulation may not be possible in all cases. See +individual instruction notes for further information. + +Supported legacy instructions +----------------------------- +* SWP{B} +Node: /proc/sys/abi/swp +Status: Obsolete +Default: Undef (0) + +* CP15 Barriers +Node: /proc/sys/abi/cp15_barrier +Status: Deprecated +Default: Emulate (1) + +* SETEND +Node: /proc/sys/abi/setend +Status: Deprecated +Default: Emulate (1)* +Note: All the cpus on the system must have mixed endian support at EL0 +for this feature to be enabled. If a new CPU - which doesn't support mixed +endian - is hotplugged in after this feature has been enabled, there could +be unexpected results in the application. |