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From:   Yury Norov <yury.norov@gmail.com>
To:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        Alan Modra <amodra@au1.ibm.com>, Daniel Axtens <dja@axtens.net>
Cc:     Yury Norov <yury.norov@gmail.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: [PATCH] powerpc/64: don't refer nr_cpu_ids in asm code when it's undefined
Date:   Tue, 20 Sep 2022 08:29:35 -0700
Message-Id: <20220920152935.1569605-1-yury.norov@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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
  ...

The [1] makes declaration of nr_cpu_ids conditional on NR_CPUS == 1,
and that reveals the issue: 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.

The issue was spotted after applying [1], which adds a CONFIG_FORCE_NR_CPUS
option that has the similar effect on nr_cpu_ids. So, in this patch, make
the LOAD_REG() conditional on CONFIG_FORCE_NR_CPUS too.

On top of:
[1] https://lore.kernel.org/lkml/20220905230820.3295223-4-yury.norov@gmail.com/T/#m96ffe122721893471fd3470d911a8f2fad6d03b3

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 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 cf2c08902c05..7cb97881635e 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -400,7 +400,11 @@ 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       */
+#endif
 	lwz	r7,0(r7)		/* also the max paca allocated 	 */
 	li	r5,0			/* logical cpu id                */
 1:
-- 
2.34.1