[PATCH v2] powerpc/64: don't refer nr_cpu_ids in asm code when it's undefined

[Yury Norov]

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@xxxxxxxxx/T/#m96ffe122721893471fd3470d911a8f2fad6d03b3

Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---
v2: move "lwz     r7,0(r7)" under #else conditional.

 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..d36939029701 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -400,8 +400,12 @@ 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 */
-- 
2.34.1