arch/arm64/asymmetric-32bit.rst

2 Asymmetric 32-bit SoCs
7 This document describes the impact of asymmetric 32-bit SoCs on the
8 execution of 32-bit (``AArch32``) applications.
16 of the CPUs are capable of executing 32-bit user applications. On such
19 ``execve(2)`` of 32-bit ELF binaries, with the latter returning
21 64-bit-only CPU, then the onlining operation fails and the new CPU is
25 running legacy 32-bit binaries. Unsurprisingly, that doesn't work very
28 It seems inevitable that future SoCs will drop 32-bit support
30 run 32-bit code on one of these transitionary platforms then you would
38 allowing 32-bit tasks to run on an asymmetric 32-bit system requires an
43 system* to mean an asymmetric 32-bit SoC running Linux with this kernel
49 32-bit tasks running on an asymmetric system behave in mostly the same
56 The subset of CPUs capable of running 32-bit tasks is described in
61 late-onlining of 32-bit-capable CPUs can result in the file contents
70 specifically when the new program being executed is 32-bit yet the
71 affinity mask contains 64-bit-only CPUs. In this situation, the kernel
74   1. If the 32-bit-capable subset of the affinity mask is not empty,
77      preserved across ``execve(2)`` of 32-bit programs.
83      ancestor is found containing at least one 32-bit-capable CPU. The
84      affinity of the task is then changed to match the 32-bit-capable
88      of all 32-bit-capable CPUs of which the kernel is aware.
90 A subsequent ``execve(2)`` of a 64-bit program by the 32-bit task will
97 Calls to ``sched_setaffinity(2)`` for a 32-bit task will consider only
98 the 32-bit-capable CPUs of the requested affinity mask. On success, the
105 Explicit admission of a 32-bit deadline task to the default root domain
107 32-bit system unless admission control is disabled by writing -1 to
110 ``execve(2)`` of a 32-bit program from a 64-bit deadline task will
112 64-bit-only CPUs and admission control is enabled. Concurrent offlining
113 of 32-bit-capable CPUs may still necessitate the procedure described in
117 **Note:** It is recommended that a set of 32-bit-capable CPUs are placed
119 32-bit tasks on an asymmetric system. Failure to do so is likely to
125 The affinity of a 32-bit task on an asymmetric system may include CPUs
129   - A 64-bit task attached to a cpuset which allows only 64-bit CPUs
130     executes a 32-bit program.
132   - All of the 32-bit-capable CPUs allowed by a cpuset containing a
133     32-bit task are offlined.
142 On an asymmetric system, the first detected 32-bit-capable CPU is
145 primary CPU (i.e. CPU 0) is 64-bit-only.
150 Although KVM will not advertise 32-bit EL0 support to any vCPUs on an
152 32-bit code at EL0. In this case, an exit from a vCPU thread in 32-bit