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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 10:05:51 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 10:05:51 +0000
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+===================
+Legacy instructions
+===================
+
+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+:Status: Obsolete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.