From b693d0b372afb39432e1c49ad7b3454855bc6bed Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Wed, 12 Jun 2019 14:52:38 -0300 Subject: docs: arm64: convert docs to ReST and rename to .rst The documentation is in a format that is very close to ReST format. The conversion is actually: - add blank lines in order to identify paragraphs; - fixing tables markups; - adding some lists markups; - marking literal blocks; - adjust some title markups. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Jonathan Corbet --- Documentation/arm64/legacy_instructions.rst | 68 +++++++++++++++++++++++++++++ 1 file changed, 68 insertions(+) create mode 100644 Documentation/arm64/legacy_instructions.rst (limited to 'Documentation/arm64/legacy_instructions.rst') diff --git a/Documentation/arm64/legacy_instructions.rst b/Documentation/arm64/legacy_instructions.rst new file mode 100644 index 000000000000..54401b22cb8f --- /dev/null +++ b/Documentation/arm64/legacy_instructions.rst @@ -0,0 +1,68 @@ +=================== +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. -- cgit