Subject: Re: [PATCH v8 15/19] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system

From: Mark Rutland <[email-masked]>
Date: Thu, 3 Jun 2021 13:58:56 +0100

On Wed, Jun 02, 2021 at 05:47:15PM +0100, Will Deacon wrote:
> If we want to support 32-bit applications, then when we identify a CPU
> with mismatched 32-bit EL0 support we must ensure that we will always
> have an active 32-bit CPU available to us from then on. This is important
> for the scheduler, because is_cpu_allowed() will be constrained to 32-bit
> CPUs for compat tasks and forced migration due to a hotplug event will
> hang if no 32-bit CPUs are available.
> 
> On detecting a mismatch, prevent offlining of either the mismatching CPU
> if it is 32-bit capable, or find the first active 32-bit capable CPU
> otherwise.
> 
> Reviewed-by: Catalin Marinas <[email-masked]>
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  arch/arm64/kernel/cpufeature.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 4194a47de62d..b31d7a1eaed6 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2877,15 +2877,33 @@ void __init setup_cpu_features(void)
>  
>  static int enable_mismatched_32bit_el0(unsigned int cpu)
>  {
> +	static int lucky_winner = -1;

This is cute, but could we please give it a meaningful name, e.g.
`pinned_cpu` ?

> +
>  	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;
> +
> +	if (lucky_winner >= 0)
> +		return 0;
> +
> +	/*
> +	 * We've detected a mismatch. We need to keep one of our CPUs with
> +	 * 32-bit EL0 online so that is_cpu_allowed() doesn't end up rejecting
> +	 * every CPU in the system for a 32-bit task.
> +	 */
> +	lucky_winner = cpu_32bit ? cpu : cpumask_any_and(cpu_32bit_el0_mask,
> +							 cpu_active_mask);
> +	get_cpu_device(lucky_winner)->offline_disabled = true;
> +	setup_elf_hwcaps(compat_elf_hwcaps);
> +	pr_info("Asymmetric 32-bit EL0 support detected on CPU %u; CPU hot-unplug disabled on CPU %u\n",
> +		cpu, lucky_winner);
>  	return 0;
>  }

I guess this is going to play havoc with kexec and hibernate. :/

Thanks,
Mark.

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