[PATCH v3] arm64: smp: Do not mark secondary CPUs possible under nosmp

[Pengjie Zhang]

Under nosmp (maxcpus=0), arm64 never brings up secondary CPUs.

smp_prepare_cpus() already treats this as a UP-mandated boot and returns
before marking secondary CPUs present. However, smp_init_cpus() may still
enumerate firmware-described secondary CPUs and mark them possible before
that point.

Avoid marking secondary CPUs possible when nosmp/maxcpus=0 is in effect,
so that cpu_possible_mask reflects the nosmp boot policy for this boot.

Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Pengjie Zhang <zhangpengjie2@xxxxxxxxxx>
---
Changes in v3:
- Tone down the changelog
- Add Catalin's Reviewed-by tag.
Link to v2:https://lore.kernel.org/all/20260423134654.4178271-1-zhangpengjie2@xxxxxxxxxx/

Changes in v2:
- Drop the arch_disable_smp_support() approach.
- Handle nosmp/maxcpus=0 directly in smp_init_cpus().
- Update the changelog accordingly.
Link to v1:https://lore.kernel.org/all/20260422095831.2926775-1-zhangpengjie2@xxxxxxxxxx/
---
 arch/arm64/kernel/smp.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 1aa324104afb..1b63846f646a 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -745,15 +745,21 @@ void __init smp_init_cpus(void)
 	else
 		acpi_parse_and_init_cpus();
 
-	if (cpu_count > nr_cpu_ids)
-		pr_warn("Number of cores (%d) exceeds configured maximum of %u - clipping\n",
-			cpu_count, nr_cpu_ids);
-
 	if (!bootcpu_valid) {
 		pr_err("missing boot CPU MPIDR, not enabling secondaries\n");
 		return;
 	}
 
+	/*
+	 * For the nosmp/maxcpus=0 case, do not mark the secondary CPUs
+	 * possible.
+	 */
+	if (!setup_max_cpus)
+		return;
+
+	if (cpu_count > nr_cpu_ids)
+		pr_warn("Number of cores (%d) exceeds configured maximum of %u - clipping\n",
+			cpu_count, nr_cpu_ids);
 	/*
 	 * We need to set the cpu_logical_map entries before enabling
 	 * the cpus so that cpu processor description entries (DT cpu nodes
-- 
2.33.0