Documentation/arch/arm64/asymmetric-32bit.rst

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======================
Asymmetric 32-bit SoCs
======================

Author: Will Deacon <will@kernel.org>

This document describes the impact of asymmetric 32-bit SoCs on the
execution of 32-bit (``AArch32``) applications.

Date: 2021-05-17

Introduction
============

Some Armv9 SoCs suffer from a big.LITTLE misfeature where only a subset
of the CPUs are capable of executing 32-bit user applications. On such
a system, Linux by default treats the asymmetry as a "mismatch" and
disables support for both the ``PER_LINUX32`` personality and
``execve(2)`` of 32-bit ELF binaries, with the latter returning
``-ENOEXEC``. If the mismatch is detected during late onlining of a
64-bit-only CPU, then the onlining operation fails and the new CPU is
unavailable for scheduling.

Surprisingly, these SoCs have been produced with the intention of
running legacy 32-bit binaries. Unsurprisingly, that doesn't work very
well with the default behaviour of Linux.

It seems inevitable that future SoCs will drop 32-bit support
altogether, so if you're stuck in the unenviable position of needing to
run 32-bit code on one of these transitionary platforms then you would
be wise to consider alternatives such as recompilation, emulation or
retirement. If neither of those options are practical, then read on.

Enabling kernel support
=======================

Since the kernel support is not completely transparent to userspace,
allowing 32-bit tasks to run on an asymmetric 32-bit system requires an
explicit "opt-in" and can be enabled by passing the
``allow_mismatched_32bit_el0`` parameter on the kernel command-line.

For the remainder of this document we will refer to an *asymmetric
system* to mean an asymmetric 32-bit SoC running Linux with this kernel
command-line option enabled.

Userspace impact
================

32-bit tasks running on an asymmetric system behave in mostly the same
way as on a homogeneous system, with a few key differences relating to
CPU affinity.

sysfs
-----

The subset of CPUs capable of running 32-bit tasks is described in
``/sys/devices/system/cpu/aarch32_el0`` and is documented further in
Documentation/ABI/testing/sysfs-devices-system-cpu.

**Note:** CPUs are advertised by this file as they are detected and so
late-onlining of 32-bit-capable CPUs can result in the file contents
being modified by the kernel at runtime. Once advertised, CPUs are never
removed from the file.

``execve(2)``
-------------

On a homogeneous system, the CPU affinity of a task is preserved across
``execve(2)``. This is not always possible on an asymmetric system,
specifically when the new program being executed is 32-bit yet the

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