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

Hi Yury,

On Tue, 20 Sep 2022 08:29:35 -0700 Yury Norov <yury.norov@gmail.com> wrote:
>

> 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
> 

I don't know PPC assembly very well, but should the #endif be one line
lower so that the constant is not dereferenced in the non-#else case? 

-- 
Cheers,
Stephen Rothwell

On Wed, Sep 21, 2022 at 08:20:06AM +1000, Stephen Rothwell wrote:
> Hi Yury,
> 
> On Tue, 20 Sep 2022 08:29:35 -0700 Yury Norov <yury.norov@gmail.com> wrote:
> >
> 
> > 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
> > 
> 
> I don't know PPC assembly very well, but should the #endif be one line
> lower so that the constant is not dereferenced in the non-#else case? 

Looks like you're right. Thanks, I'll send a v2.