path: root/Documentation
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authorPunit Agrawal <punit.agrawal@arm.com>2014-11-18 11:41:24 +0000
committerDavid Brown <david.brown@linaro.org>2015-12-03 16:53:43 -0800
commite1cdd62cd18908c503eabf498b17996ed26e4e00 (patch)
treefd8abd392b0c2d222cc9ac6c1a19766e3642b43e /Documentation
parentdf532aaee10d56eefc636f0f45660bec6a2806f8 (diff)
arm64: Add framework for legacy instruction emulation
commit 587064b610c703f259317d00dc37bf6d40f4fc74 upstream. Typically, providing support for legacy instructions requires emulating the behaviour of instructions whose encodings have become undefined. If the instructions haven't been removed from the architecture, there maybe an option in the implementation to turn on/off the support for these instructions. Create common infrastructure to support legacy instruction emulation. In addition to emulation, also provide an option to support hardware execution when supported. The default execution mode (one of undef, emulate, hw exeuction) is dependent on the state of the instruction (deprecated or obsolete) in the architecture and can specified at the time of registering the instruction handlers. The runtime state of the emulation can be controlled by writing to individual nodes in sysctl. The expected default behaviour is documented as part of this patch. Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: David Brown <david.brown@linaro.org>
Diffstat (limited to 'Documentation')
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diff --git a/Documentation/arm64/legacy_instructions.txt b/Documentation/arm64/legacy_instructions.txt
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+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.