[PATCH] arm64: smp: Limit nr_cpu_ids under nosmp

[Pengjie Zhang]

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);
+}
+
 void __init smp_cpus_done(unsigned int max_cpus)
 {
 	pr_info("SMP: Total of %d processors activated.\n", num_online_cpus());
-- 
2.33.0