[PATCH v6 18/21] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system

[Will Deacon]

Hi Catalin,

On Mon, May 24, 2021 at 04:46:58PM +0100, Catalin Marinas wrote:
> On Tue, May 18, 2021 at 10:47:22AM +0100, Will Deacon wrote:
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 959442f76ed7..72efdc611b14 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -2896,15 +2896,33 @@ void __init setup_cpu_features(void)
> >  
> >  static int enable_mismatched_32bit_el0(unsigned int cpu)
> >  {
> > +	static int lucky_winner = -1;
> > +
> >  	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> >  	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> >  
> >  	if (cpu_32bit) {
> >  		cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
> >  		static_branch_enable_cpuslocked(&arm64_mismatched_32bit_el0);
> > -		setup_elf_hwcaps(compat_elf_hwcaps);
> >  	}
> >  
> > +	if (cpumask_test_cpu(0, cpu_32bit_el0_mask) == cpu_32bit)
> > +		return 0;
> 
> I don't fully understand this early return. AFAICT, we still call
> setup_elf_hwcaps() via setup_cpu_features() if the system supports
> 32-bit EL0 (mismatched or not) at boot. For CPU hotplug, we can add the
> compat hwcaps later if we didn't set them up at boot. So this part is
> fine.
> 
> However, if CPU0 is 32-bit-capable, it looks like we'd never disable the
> offlining on any of the 32-bit-capable CPUs and there's nothing that
> prevents offlining CPU0.

That is also deferred until we actually detect the mismatch. For example, if
CPU0 is 32-bit capable but none of the others are, then when we online CPU1
we will print:

  | CPU features: Asymmetric 32-bit EL0 support detected on CPU 1; CPU hot-unplug disabled on CPU 0

so the check above is really asking "Is the CPU being onlined mismatched wrt
the boot CPU?". If yes, then we need to make sure that we're keeping a
32-bit-capable CPU around.

Will