[PATCH v2] arm64: smp: Do not mark secondary CPUs possible under nosmp

[PATCH v2] arm64: smp: Do not mark secondary CPUs possible under nosmp

[Pengjie Zhang]

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

However, arm64 still enumerates firmware-described CPUs during SMP
initialization, which can leave secondary CPUs 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.

Return early from smp_init_cpus() when nosmp/maxcpus=0 is in effect
so that secondary CPUs are not marked possible on arm64.

Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
---
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@huawei.com/

---
 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

Re: [PATCH v2] arm64: smp: Do not mark secondary CPUs possible under nosmp

[Catalin Marinas]

On Thu, Apr 23, 2026 at 09:46:54PM +0800, Pengjie Zhang wrote:
> Under nosmp (maxcpus=0), arm64 never brings up secondary CPUs.
> 
> However, arm64 still enumerates firmware-described CPUs during SMP
> initialization, which can leave secondary CPUs 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.

I'm fine with the patch in principle but I fail to see why it is not
mostly cosmetic. If we have possible & !present CPUs (there's another
thread around cpuhp_smt_enable() to allow this combination on arm64),
get_cpu_device() would return NULL and the core code is supposed to
handle this. What other per-CPU state should be initialised for a
possible CPU but it is not without this patch?

-- 
Catalin