[PATCH] arm64: smp: Limit nr_cpu_ids under nosmp

[Catalin Marinas]

On Wed, Apr 22, 2026 at 05:58:31PM +0800, Pengjie Zhang wrote:
> Under nosmp (maxcpus=0), arm64 never brings up secondary CPUs.
> 
> However, arm64 still enumerates firmware-described CPUs during SMP
> initialization, so secondary CPUs can remain visible to
> for_each_possible_cpu() users even though they never reach the
> bringup path in this configuration.
> 
> This is not just a cosmetic mask mismatch: code iterating over
> possible CPUs may observe secondary CPU per-CPU state that is never
> fully initialized under nosmp.
> 
> Limit nr_cpu_ids to 1 in arch_disable_smp_support() so that
> secondary CPUs are not set up on arm64 when nosmp/maxcpus=0 is in
> effect.
> 
> Signed-off-by: Pengjie Zhang <zhangpengjie2 at huawei.com>
> ---
>  arch/arm64/kernel/smp.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 1aa324104afb..cc34c68871e9 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -435,6 +435,15 @@ static void __init hyp_mode_check(void)
>  	}
>  }
>  
> +void __init arch_disable_smp_support(void)
> +{
> +	/*
> +	 * Under nosmp/maxcpus=0, only the boot CPU can ever be brought up.
> +	 * Limit nr_cpu_ids so that secondary CPUs are never set up.
> +	 */
> +	set_nr_cpu_ids(1);
> +}

I don't think that's the right fix. We don't have anything like the x86
ioapic to disable in this function, so no need to implement it. If
nr_cpu_ids must be 1 with nosmp/maxcpus=0, I'd rather do this in the
generic code. It need some alignment with other architectures if we are
to do this early. IOW, is nosmp equivalent to nr_cpus=1?

In the meantime, for arm64, we can do something like below and let the
generic code set nr_cpu_ids() via start_kernel() -> setup_nr_cpu_ids().

-------------8<-------------------
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 1aa324104afb..7364481cc03a 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -754,6 +754,13 @@ void __init smp_init_cpus(void)
 		return;
 	}

+	/*
+	 * For the nosmp/maxcpus=0 case, do not mark the secondary CPUs
+	 * possible.
+	 */
+	if (!setup_max_cpus)
+		return;
+
 	/*
 	 * We need to set the cpu_logical_map entries before enabling
 	 * the cpus so that cpu processor description entries (DT cpu nodes