Documentation/arm64/legacy_instructions.txt

   1 The arm64 port of the Linux kernel provides infrastructure to support
   2 emulation of instructions which have been deprecated, or obsoleted in
   3 the architecture. The infrastructure code uses undefined instruction
   4 hooks to support emulation. Where available it also allows turning on
   5 the instruction execution in hardware.
   6
   7 The emulation mode can be controlled by writing to sysctl nodes
   8 (/proc/sys/abi). The following explains the different execution
   9 behaviours and the corresponding values of the sysctl nodes -
  10
  11 * Undef
  12   Value: 0
  13   Generates undefined instruction abort. Default for instructions that
  14   have been obsoleted in the architecture, e.g., SWP
  15
  16 * Emulate
  17   Value: 1
  18   Uses software emulation. To aid migration of software, in this mode
  19   usage of emulated instruction is traced as well as rate limited
  20   warnings are issued. This is the default for deprecated
  21   instructions, .e.g., CP15 barriers
  22
  23 * Hardware Execution
  24   Value: 2
  25   Although marked as deprecated, some implementations may support the
  26   enabling/disabling of hardware support for the execution of these
  27   instructions. Using hardware execution generally provides better
  28   performance, but at the loss of ability to gather runtime statistics
  29   about the use of the deprecated instructions.
  30
  31 The default mode depends on the status of the instruction in the
  32 architecture. Deprecated instructions should default to emulation
  33 while obsolete instructions must be undefined by default.
  34
  35 Note: Instruction emulation may not be possible in all cases. See
  36 individual instruction notes for further information.
  37
  38 Supported legacy instructions
  39 -----------------------------
  40 * SWP{B}
  41 Node: /proc/sys/abi/swp
  42 Status: Obsolete
  43 Default: Undef (0)
  44
  45 * CP15 Barriers
  46 Node: /proc/sys/abi/cp15_barrier
  47 Status: Deprecated
  48 Default: Emulate (1)
  49
  50 * SETEND
  51 Node: /proc/sys/abi/setend
  52 Status: Deprecated
  53 Default: Emulate (1)*
  54 Note: All the cpus on the system must have mixed endian support at EL0
  55 for this feature to be enabled. If a new CPU - which doesn't support mixed
  56 endian - is hotplugged in after this feature has been enabled, there could
  57 be unexpected results in the application.