projects / mirror_ubuntu-jammy-kernel.git / blame
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|---|---|---|
| b693d0b3 MCC |
1 | =================== |
| 2 | Legacy instructions | |
| 3 | =================== | |
| 4 | ||
| 587064b6 PA |
5 | The arm64 port of the Linux kernel provides infrastructure to support |
| 6 | emulation of instructions which have been deprecated, or obsoleted in | |
| 7 | the architecture. The infrastructure code uses undefined instruction | |
| 8 | hooks to support emulation. Where available it also allows turning on | |
| 9 | the instruction execution in hardware. | |
| 10 | ||
| 11 | The emulation mode can be controlled by writing to sysctl nodes | |
| 12 | (/proc/sys/abi). The following explains the different execution | |
| 13 | behaviours and the corresponding values of the sysctl nodes - | |
| 14 | ||
| 15 | * Undef | |
| b693d0b3 MCC |
16 | Value: 0 |
| 17 | ||
| 587064b6 PA |
18 | Generates undefined instruction abort. Default for instructions that |
| 19 | have been obsoleted in the architecture, e.g., SWP | |
| 20 | ||
| 21 | * Emulate | |
| b693d0b3 MCC |
22 | Value: 1 |
| 23 | ||
| 587064b6 PA |
24 | Uses software emulation. To aid migration of software, in this mode |
| 25 | usage of emulated instruction is traced as well as rate limited | |
| 26 | warnings are issued. This is the default for deprecated | |
| 27 | instructions, .e.g., CP15 barriers | |
| 28 | ||
| 29 | * Hardware Execution | |
| b693d0b3 MCC |
30 | Value: 2 |
| 31 | ||
| 587064b6 PA |
32 | Although marked as deprecated, some implementations may support the |
| 33 | enabling/disabling of hardware support for the execution of these | |
| 34 | instructions. Using hardware execution generally provides better | |
| 35 | performance, but at the loss of ability to gather runtime statistics | |
| 36 | about the use of the deprecated instructions. | |
| 37 | ||
| 38 | The default mode depends on the status of the instruction in the | |
| 39 | architecture. Deprecated instructions should default to emulation | |
| 40 | while obsolete instructions must be undefined by default. | |
| bd35a4ad | 41 | |
| 2d888f48 SP |
42 | Note: Instruction emulation may not be possible in all cases. See |
| 43 | individual instruction notes for further information. | |
| 44 | ||
| bd35a4ad PA |
45 | Supported legacy instructions |
| 46 | ----------------------------- | |
| 47 | * SWP{B} | |
| b693d0b3 MCC |
48 | |
| 49 | :Node: /proc/sys/abi/swp | |
| 50 | :Status: Obsolete | |
| 51 | :Default: Undef (0) | |
| c852f320 PA |
52 | |
| 53 | * CP15 Barriers | |
| b693d0b3 MCC |
54 | |
| 55 | :Node: /proc/sys/abi/cp15_barrier | |
| 56 | :Status: Deprecated | |
| 57 | :Default: Emulate (1) | |
| 2d888f48 SP |
58 | |
| 59 | * SETEND | |
| b693d0b3 MCC |
60 | |
| 61 | :Node: /proc/sys/abi/setend | |
| 62 | :Status: Deprecated | |
| 63 | :Default: Emulate (1)* | |
| 64 | ||
| 65 | Note: All the cpus on the system must have mixed endian support at EL0 | |
| 66 | for this feature to be enabled. If a new CPU - which doesn't support mixed | |
| 67 | endian - is hotplugged in after this feature has been enabled, there could | |
| 68 | be unexpected results in the application. |