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diff --git a/Documentation/arm64/asymmetric-32bit.rst b/Documentation/arm64/asymmetric-32bit.rst new file mode 100644 index 000000000..64a0b505d --- /dev/null +++ b/Documentation/arm64/asymmetric-32bit.rst @@ -0,0 +1,155 @@ +====================== +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 +affinity mask contains 64-bit-only CPUs. In this situation, the kernel +determines the new affinity mask as follows: + + 1. If the 32-bit-capable subset of the affinity mask is not empty, + then the affinity is restricted to that subset and the old affinity + mask is saved. This saved mask is inherited over ``fork(2)`` and + preserved across ``execve(2)`` of 32-bit programs. + + **Note:** This step does not apply to ``SCHED_DEADLINE`` tasks. + See `SCHED_DEADLINE`_. + + 2. Otherwise, the cpuset hierarchy of the task is walked until an + ancestor is found containing at least one 32-bit-capable CPU. The + affinity of the task is then changed to match the 32-bit-capable + subset of the cpuset determined by the walk. + + 3. On failure (i.e. out of memory), the affinity is changed to the set + of all 32-bit-capable CPUs of which the kernel is aware. + +A subsequent ``execve(2)`` of a 64-bit program by the 32-bit task will +invalidate the affinity mask saved in (1) and attempt to restore the CPU +affinity of the task using the saved mask if it was previously valid. +This restoration may fail due to intervening changes to the deadline +policy or cpuset hierarchy, in which case the ``execve(2)`` continues +with the affinity unchanged. + +Calls to ``sched_setaffinity(2)`` for a 32-bit task will consider only +the 32-bit-capable CPUs of the requested affinity mask. On success, the +affinity for the task is updated and any saved mask from a prior +``execve(2)`` is invalidated. + +``SCHED_DEADLINE`` +------------------ + +Explicit admission of a 32-bit deadline task to the default root domain +(e.g. by calling ``sched_setattr(2)``) is rejected on an asymmetric +32-bit system unless admission control is disabled by writing -1 to +``/proc/sys/kernel/sched_rt_runtime_us``. + +``execve(2)`` of a 32-bit program from a 64-bit deadline task will +return ``-ENOEXEC`` if the root domain for the task contains any +64-bit-only CPUs and admission control is enabled. Concurrent offlining +of 32-bit-capable CPUs may still necessitate the procedure described in +`execve(2)`_, in which case step (1) is skipped and a warning is +emitted on the console. + +**Note:** It is recommended that a set of 32-bit-capable CPUs are placed +into a separate root domain if ``SCHED_DEADLINE`` is to be used with +32-bit tasks on an asymmetric system. Failure to do so is likely to +result in missed deadlines. + +Cpusets +------- + +The affinity of a 32-bit task on an asymmetric system may include CPUs +that are not explicitly allowed by the cpuset to which it is attached. +This can occur as a result of the following two situations: + + - A 64-bit task attached to a cpuset which allows only 64-bit CPUs + executes a 32-bit program. + + - All of the 32-bit-capable CPUs allowed by a cpuset containing a + 32-bit task are offlined. + +In both of these cases, the new affinity is calculated according to step +(2) of the process described in `execve(2)`_ and the cpuset hierarchy is +unchanged irrespective of the cgroup version. + +CPU hotplug +----------- + +On an asymmetric system, the first detected 32-bit-capable CPU is +prevented from being offlined by userspace and any such attempt will +return ``-EPERM``. Note that suspend is still permitted even if the +primary CPU (i.e. CPU 0) is 64-bit-only. + +KVM +--- + +Although KVM will not advertise 32-bit EL0 support to any vCPUs on an +asymmetric system, a broken guest at EL1 could still attempt to execute +32-bit code at EL0. In this case, an exit from a vCPU thread in 32-bit +mode will return to host userspace with an ``exit_reason`` of +``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully +re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation. |