Lines Matching +full:mode +full:- +full:capable
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.
10 Date: 2021-05-17
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
20 ``-ENOEXEC``. If the mismatch is detected during late onlining of a
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
39 explicit "opt-in" and can be enabled by passing the
40 ``allow_mismatched_32bit_el0`` parameter on the kernel command-line.
43 system* to mean an asymmetric 32-bit SoC running Linux with this kernel
44 command-line option enabled.
49 32-bit tasks running on an asymmetric system behave in mostly the same
54 -----
56 The subset of CPUs capable of running 32-bit tasks is described in
58 ``Documentation/ABI/testing/sysfs-devices-system-cpu``.
61 late-onlining of 32-bit-capable CPUs can result in the file contents
66 -------------
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
103 ------------------
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
111 return ``-ENOEXEC`` if the root domain for the task contains any
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
123 -------
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.
140 -----------
142 On an asymmetric system, the first detected 32-bit-capable CPU is
144 return ``-EPERM``. Note that suspend is still permitted even if the
145 primary CPU (i.e. CPU 0) is 64-bit-only.
148 ---
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
153 mode will return to host userspace with an ``exit_reason`` of
154 ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
155 re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.