Re: [PATCH 1/3] ARM: Pass cpu number to secondary_start_kernel

[Ard Biesheuvel]

Hi Keith,

This looks like a worthwhile cleanup to me regardless of the following patches.

On Sat, 4 Sept 2021 at 08:09, Keith Packard <keithp@xxxxxxxxxx> wrote:
>
> Instead of pulling the CPU value out of the thread_info struct, pass
> it as an argument. When first initializing secondary processors, this
> is done by stashing the value in the secondary_data struct. When
> restarting idle processors, the previous CPU value is passed.
>
> Because the cpu is now known at the top of secondary_start_kernel, the
> per_cpu_offset value can now be set at this point, instead of in
> cpu_init where it was also incorrectly setting the per_cpu_offset for
> the boot processor before that value had been computed.
>
> Signed-off-by: Keith Packard <keithpac@xxxxxxxxxx>

Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> ---
>  arch/arm/include/asm/smp.h   |  3 ++-
>  arch/arm/kernel/head-nommu.S |  1 +
>  arch/arm/kernel/head.S       |  1 +
>  arch/arm/kernel/setup.c      |  6 ------
>  arch/arm/kernel/smp.c        | 14 +++++++++-----
>  5 files changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
> index 5d508f5d56c4..86a7fd721556 100644
> --- a/arch/arm/include/asm/smp.h
> +++ b/arch/arm/include/asm/smp.h
> @@ -48,7 +48,7 @@ extern void set_smp_ipi_range(int ipi_base, int nr_ipi);
>   * Called from platform specific assembly code, this is the
>   * secondary CPU entry point.
>   */
> -asmlinkage void secondary_start_kernel(void);
> +asmlinkage void secondary_start_kernel(unsigned int cpu);
>
>
>  /*
> @@ -61,6 +61,7 @@ struct secondary_data {
>         };
>         unsigned long swapper_pg_dir;
>         void *stack;
> +       unsigned int cpu;
>  };
>  extern struct secondary_data secondary_data;
>  extern void secondary_startup(void);
> diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
> index 0fc814bbc34b..5aa8ef42717f 100644
> --- a/arch/arm/kernel/head-nommu.S
> +++ b/arch/arm/kernel/head-nommu.S
> @@ -114,6 +114,7 @@ ENTRY(secondary_startup)
>         add     r12, r12, r10
>         ret     r12
>  1:     bl      __after_proc_init
> +       ldr     r0, [r7, #16]                   @ set up cpu number
>         ldr     sp, [r7, #12]                   @ set up the stack pointer
>         mov     fp, #0
>         b       secondary_start_kernel
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 7f62c5eccdf3..0e541af738e2 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -394,6 +394,7 @@ ENDPROC(secondary_startup_arm)
>
>  ENTRY(__secondary_switched)
>         ldr_l   r7, secondary_data + 12         @ get secondary_data.stack
> +       ldr_l   r0, secondary_data + 16         @ get secondary_data.cpu
>         mov     sp, r7
>         mov     fp, #0
>         b       secondary_start_kernel
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 73ca7797b92f..ca0201635fac 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -532,12 +532,6 @@ void notrace cpu_init(void)
>                 BUG();
>         }
>
> -       /*
> -        * This only works on resume and secondary cores. For booting on the
> -        * boot cpu, smp_prepare_boot_cpu is called after percpu area setup.
> -        */
> -       set_my_cpu_offset(per_cpu_offset(cpu));
> -
>         cpu_proc_init();
>
>         /*
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 74679240a9d8..55cb1689a4b3 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -153,6 +153,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
>         secondary_data.pgdir = virt_to_phys(idmap_pgd);
>         secondary_data.swapper_pg_dir = get_arch_pgd(swapper_pg_dir);
>  #endif
> +       secondary_data.cpu = cpu;
>         sync_cache_w(&secondary_data);
>
>         /*
> @@ -373,11 +374,14 @@ void arch_cpu_idle_dead(void)
>          * cpu initialisation.  There's some initialisation which needs
>          * to be repeated to undo the effects of taking the CPU offline.
>          */
> -       __asm__("mov    sp, %0\n"
> +       __asm__("mov    r0, %1\n"
> +       "       mov     sp, %0\n"
>         "       mov     fp, #0\n"
>         "       b       secondary_start_kernel"
>                 :
> -               : "r" (task_stack_page(current) + THREAD_SIZE - 8));
> +               : "r" (task_stack_page(current) + THREAD_SIZE - 8),
> +                 "r" (cpu)
> +               : "r0");
>  }
>  #endif /* CONFIG_HOTPLUG_CPU */
>
> @@ -400,10 +404,11 @@ static void smp_store_cpu_info(unsigned int cpuid)
>   * This is the secondary CPU boot entry.  We're using this CPUs
>   * idle thread stack, but a set of temporary page tables.
>   */
> -asmlinkage void secondary_start_kernel(void)
> +asmlinkage void secondary_start_kernel(unsigned int cpu)
>  {
>         struct mm_struct *mm = &init_mm;
> -       unsigned int cpu;
> +
> +       set_my_cpu_offset(per_cpu_offset(cpu));
>
>         secondary_biglittle_init();
>
> @@ -420,7 +425,6 @@ asmlinkage void secondary_start_kernel(void)
>          * All kernel threads share the same mm context; grab a
>          * reference and switch to it.
>          */
> -       cpu = smp_processor_id();
>         mmgrab(mm);
>         current->active_mm = mm;
>         cpumask_set_cpu(cpu, mm_cpumask(mm));
> --
> 2.33.0
>