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

[Will Deacon]

On Thu, Nov 19, 2020 at 05:11:27PM +0100, Peter Zijlstra wrote:
> 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?

See my other reply, but there are 64-bit apps that execve() a 32-bit
payload. I was hoping this could be handling in the C library, but that
has no idea about what it's exec'ing beyond the path.

Will