linux-kernel - [PATCH AUTOSEL 5.10 09/16] powerpc/64: don't refer nr_cpu

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Message-Id: <20221018001029.2731620-9-sashal@kernel.org>
Date:   Mon, 17 Oct 2022 20:10:22 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Sasha Levin <sashal@...nel.org>, mpe@...erman.id.au,
        aik@...abs.ru, christophe.leroy@...roup.eu, dja@...ens.net,
        amodra@....ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 5.10 09/16] powerpc/64: don't refer nr_cpu_ids in asm code when it's undefined

From: Yury Norov <yury.norov@...il.com>

[ Upstream commit 546a073d628111e3338af689938407e77d5dc38f ]

generic_secondary_common_init() calls LOAD_REG_ADDR(r7, nr_cpu_ids)
conditionally on CONFIG_SMP. However, if 'NR_CPUS == 1', kernel doesn't
use the nr_cpu_ids, and in C code, it's just:
  #if NR_CPUS == 1
  #define nr_cpu_ids
  ...

This series makes declaration of nr_cpu_ids conditional on NR_CPUS == 1,
and that reveals the issue, because compiler can't link the
LOAD_REG_ADDR(r7, nr_cpu_ids) against nonexisting symbol.

Current code looks unsafe for those who build kernel with CONFIG_SMP=y and
NR_CPUS == 1. This is weird configuration, but not disallowed.

Fix the linker error by replacing LOAD_REG_ADDR() with LOAD_REG_IMMEDIATE()
conditionally on NR_CPUS == 1.

As the following patch adds CONFIG_FORCE_NR_CPUS option that has the
similar effect on nr_cpu_ids, make the generic_secondary_common_init()
conditional on it too.

Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Signed-off-by: Yury Norov <yury.norov@...il.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/powerpc/kernel/head_64.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 2d6581db0c7b..86f9970d8631 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -392,8 +392,12 @@ generic_secondary_common_init:
 #else
 	LOAD_REG_ADDR(r8, paca_ptrs)	/* Load paca_ptrs pointe	 */
 	ld	r8,0(r8)		/* Get base vaddr of array	 */
+#if (NR_CPUS == 1) || defined(CONFIG_FORCE_NR_CPUS)
+	LOAD_REG_IMMEDIATE(r7, NR_CPUS)
+#else
 	LOAD_REG_ADDR(r7, nr_cpu_ids)	/* Load nr_cpu_ids address       */
 	lwz	r7,0(r7)		/* also the max paca allocated 	 */
+#endif
 	li	r5,0			/* logical cpu id                */
 1:
 	sldi	r9,r5,3			/* get paca_ptrs[] index from cpu id */
-- 
2.35.1