Re: [PATCH v3 00/14] An alternative series for asymmetric AArch32 systems

[Peter Zijlstra]

On Fri, Nov 13, 2020 at 09:37:05AM +0000, Will Deacon wrote:

> The aim of this series is to allow 32-bit ARM applications to run on
> arm64 SoCs where not all of the CPUs support the 32-bit instruction set.
> 
> There are some major changes in v3:
> 
>   * Add some scheduler hooks for restricting a task's affinity mask
>   * Implement these hooks for arm64 so that we can avoid 32-bit tasks
>     running on 64-bit-only cores
>   * Restrict affinity mask of 32-bit tasks on execve()
>   * Prevent hot-unplug of all 32-bit CPUs if we have a mismatched system
>   * Ensure 32-bit EL0 cpumask is zero-initialised (oops)
> 
> It's worth mentioning that this approach goes directly against my
> initial proposal for punting the affinity management to userspace,
> because it turns out that doesn't really work. There are cases where the
> kernel has to muck with the affinity mask explicitly, such as execve(),
> CPU hotplug and cpuset balancing. Ensuring that these don't lead to
> random SIGKILLs as far as userspace is concerned means avoiding any

Mooo, I thought we were okay with that... Use does stupid, user gets
SIGKIL. What changed?

> 64-bit-only CPUs appearing in the affinity mask for a 32-bit task, at
> which point it's easier just to handle everything in the kernel anyway.