The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 777f81f0a9c780a6443bcf2c7785f0cc2e87c1ef Mon Sep 17 00:00:00 2001
From: Michael Ellerman <mpe@ellerman.id.au>
Date: Thu, 15 Feb 2024 00:14:04 +1100
Subject: [PATCH] powerpc/smp: Increase nr_cpu_ids to include the boot CPU

If nr_cpu_ids is too low to include the boot CPU adjust nr_cpu_ids
upward. Otherwise the kernel will BUG when trying to allocate a paca
for the boot CPU and fail to boot.

Cc: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231229120107.2281153-2-mpe@ellerman.id.au
---
 arch/powerpc/kernel/prom.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 58e80076bed5c..77364729a1b61 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -381,6 +381,12 @@ static int __init early_init_dt_scan_cpus(unsigned long node,
 			nr_cpu_ids);
 	}
 
+	if (boot_cpuid >= nr_cpu_ids) {
+		set_nr_cpu_ids(min(CONFIG_NR_CPUS, ALIGN(boot_cpuid + 1, nthreads)));
+		pr_warn("Boot CPU %d >= nr_cpu_ids, adjusted nr_cpu_ids to %d\n",
+			boot_cpuid, nr_cpu_ids);
+	}
+
 	/*
 	 * PAPR defines "logical" PVR values for cpus that
 	 * meet various levels of the architecture:
-- 
2.43.0