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

From: Catalin Marinas <[email-masked]>
Date: Mon, 27 Apr 2026 14:20:41 +0100

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