[RFC 3/6] RISC-V: Use __cpu_up_stack/task_pointer only for spinwait method

[Anup Patel]

On Sat, Dec 4, 2021 at 5:51 AM Atish Patra <atishp at atishpatra.org> wrote:
>
> From: Atish Patra <atishp at rivosinc.com>
>
> The __cpu_up_stack/task_pointer array is only used for spinwait method
> now. The per cpu array based lookup is also fragile for platforms with
> discontiguous/sparse hartids. The spinwait method is only used for
> M-mode Linux or older firmwares without SBI HSM extension. For general
> Linux systems, ordered booting method is preferred anyways to support
> cpu hotplug and kexec.
>
> Make sure that __cpu_up_stack/task_pointer is only used for spinwait
> method. Take this opportunity to rename it to
> __cpu_spinwait_stack/task_pointer to emphasize the purpose as well.
>
> Signed-off-by: Atish Patra <atishp at rivosinc.com>

Looks good to me.

Reviewed-by: Anup Patel <anup at brainfault.org>

Regards,
Anup

> ---
>  arch/riscv/include/asm/cpu_ops.h     |  2 --
>  arch/riscv/kernel/cpu_ops.c          | 16 ----------------
>  arch/riscv/kernel/cpu_ops_spinwait.c | 27 ++++++++++++++++++++++++++-
>  arch/riscv/kernel/head.S             |  4 ++--
>  arch/riscv/kernel/head.h             |  4 ++--
>  5 files changed, 30 insertions(+), 23 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cpu_ops.h b/arch/riscv/include/asm/cpu_ops.h
> index a8ec3c5c1bd2..134590f1b843 100644
> --- a/arch/riscv/include/asm/cpu_ops.h
> +++ b/arch/riscv/include/asm/cpu_ops.h
> @@ -40,7 +40,5 @@ struct cpu_operations {
>
>  extern const struct cpu_operations *cpu_ops[NR_CPUS];
>  void __init cpu_set_ops(int cpu);
> -void cpu_update_secondary_bootdata(unsigned int cpuid,
> -                                  struct task_struct *tidle);
>
>  #endif /* ifndef __ASM_CPU_OPS_H */
> diff --git a/arch/riscv/kernel/cpu_ops.c b/arch/riscv/kernel/cpu_ops.c
> index 3f5a38b03044..c1e30f403c3b 100644
> --- a/arch/riscv/kernel/cpu_ops.c
> +++ b/arch/riscv/kernel/cpu_ops.c
> @@ -8,31 +8,15 @@
>  #include <linux/of.h>
>  #include <linux/string.h>
>  #include <linux/sched.h>
> -#include <linux/sched/task_stack.h>
>  #include <asm/cpu_ops.h>
>  #include <asm/sbi.h>
>  #include <asm/smp.h>
>
>  const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;
>
> -void *__cpu_up_stack_pointer[NR_CPUS] __section(".data");
> -void *__cpu_up_task_pointer[NR_CPUS] __section(".data");
> -
>  extern const struct cpu_operations cpu_ops_sbi;
>  extern const struct cpu_operations cpu_ops_spinwait;
>
> -void cpu_update_secondary_bootdata(unsigned int cpuid,
> -                                  struct task_struct *tidle)
> -{
> -       int hartid = cpuid_to_hartid_map(cpuid);
> -
> -       /* Make sure tidle is updated */
> -       smp_mb();
> -       WRITE_ONCE(__cpu_up_stack_pointer[hartid],
> -                  task_stack_page(tidle) + THREAD_SIZE);
> -       WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
> -}
> -
>  void __init cpu_set_ops(int cpuid)
>  {
>  #if IS_ENABLED(CONFIG_RISCV_SBI)
> diff --git a/arch/riscv/kernel/cpu_ops_spinwait.c b/arch/riscv/kernel/cpu_ops_spinwait.c
> index b2c957bb68c1..9f398eb94f7a 100644
> --- a/arch/riscv/kernel/cpu_ops_spinwait.c
> +++ b/arch/riscv/kernel/cpu_ops_spinwait.c
> @@ -6,11 +6,36 @@
>  #include <linux/errno.h>
>  #include <linux/of.h>
>  #include <linux/string.h>
> +#include <linux/sched/task_stack.h>
>  #include <asm/cpu_ops.h>
>  #include <asm/sbi.h>
>  #include <asm/smp.h>
>
>  const struct cpu_operations cpu_ops_spinwait;
> +void *__cpu_spinwait_stack_pointer[NR_CPUS] __section(".data");
> +void *__cpu_spinwait_task_pointer[NR_CPUS] __section(".data");
> +
> +static void cpu_update_secondary_bootdata(unsigned int cpuid,
> +                                  struct task_struct *tidle)
> +{
> +       int hartid = cpuid_to_hartid_map(cpuid);
> +
> +       /*
> +        * The hartid must be less than NR_CPUS to avoid out-of-bound access
> +        * errors for __cpu_spinwait_stack/task_pointer. That is not always possible
> +        * for platforms with discontiguous hartid numbering scheme. That's why
> +        * spinwait booting is not the recommended approach for any platforms
> +        * and will be removed in future.
> +        */
> +       if (hartid == INVALID_HARTID || hartid >= NR_CPUS)
> +               return;
> +
> +       /* Make sure tidle is updated */
> +       smp_mb();
> +       WRITE_ONCE(__cpu_spinwait_stack_pointer[hartid],
> +                  task_stack_page(tidle) + THREAD_SIZE);
> +       WRITE_ONCE(__cpu_spinwait_task_pointer[hartid], tidle);
> +}
>
>  static int spinwait_cpu_prepare(unsigned int cpuid)
>  {
> @@ -28,7 +53,7 @@ static int spinwait_cpu_start(unsigned int cpuid, struct task_struct *tidle)
>          * selects the first cpu to boot the kernel and causes the remainder
>          * of the cpus to spin in a loop waiting for their stack pointer to be
>          * setup by that main cpu.  Writing to bootdata
> -        * (i.e __cpu_up_stack_pointer) signals to the spinning cpus that they
> +        * (i.e __cpu_spinwait_stack_pointer) signals to the spinning cpus that they
>          * can continue the boot process.
>          */
>         cpu_update_secondary_bootdata(cpuid, tidle);
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 40d4c625513c..6f8e99eac6a1 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -347,9 +347,9 @@ clear_bss_done:
>         csrw CSR_TVEC, a3
>
>         slli a3, a0, LGREG
> -       la a1, __cpu_up_stack_pointer
> +       la a1, __cpu_spinwait_stack_pointer
>         XIP_FIXUP_OFFSET a1
> -       la a2, __cpu_up_task_pointer
> +       la a2, __cpu_spinwait_task_pointer
>         XIP_FIXUP_OFFSET a2
>         add a1, a3, a1
>         add a2, a3, a2
> diff --git a/arch/riscv/kernel/head.h b/arch/riscv/kernel/head.h
> index aabbc3ac3e48..5393cca77790 100644
> --- a/arch/riscv/kernel/head.h
> +++ b/arch/riscv/kernel/head.h
> @@ -16,7 +16,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa);
>  asmlinkage void __init __copy_data(void);
>  #endif
>
> -extern void *__cpu_up_stack_pointer[];
> -extern void *__cpu_up_task_pointer[];
> +extern void *__cpu_spinwait_stack_pointer[];
> +extern void *__cpu_spinwait_task_pointer[];
>
>  #endif /* __ASM_HEAD_H */
> --
> 2.33.1
>