Re: [PATCH v3 3/8] RISC-V: Check scalar unaligned access on all CPUs

[Evan Green]

On Mon, Jun 24, 2024 at 5:51 PM Jesse Taube <jesse@xxxxxxxxxxxx> wrote:
>
> Originally, the check_unaligned_access_emulated_all_cpus function
> only checked the boot hart. This fixes the function to check all
> harts.
>
> Fixes: 71c54b3d169d ("riscv: report misaligned accesses emulation to hwprobe")
> Signed-off-by: Jesse Taube <jesse@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> V1 -> V2:
>  - New patch
> V2 -> V3:
>  - Split patch
> ---
>  arch/riscv/kernel/traps_misaligned.c | 23 ++++++-----------------
>  1 file changed, 6 insertions(+), 17 deletions(-)
>
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index b62d5a2f4541..8fadbe00dd62 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -526,31 +526,17 @@ int handle_misaligned_store(struct pt_regs *regs)
>         return 0;
>  }
>
> -static bool check_unaligned_access_emulated(int cpu)
> +static void check_unaligned_access_emulated(struct work_struct *unused)
>  {
> +       int cpu = smp_processor_id();
>         long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
>         unsigned long tmp_var, tmp_val;
> -       bool misaligned_emu_detected;
>
>         *mas_ptr = RISCV_HWPROBE_MISALIGNED_UNKNOWN;
>
>         __asm__ __volatile__ (
>                 "       "REG_L" %[tmp], 1(%[ptr])\n"
>                 : [tmp] "=r" (tmp_val) : [ptr] "r" (&tmp_var) : "memory");
> -
> -       misaligned_emu_detected = (*mas_ptr == RISCV_HWPROBE_MISALIGNED_EMULATED);
> -       /*
> -        * If unaligned_ctl is already set, this means that we detected that all
> -        * CPUS uses emulated misaligned access at boot time. If that changed
> -        * when hotplugging the new cpu, this is something we don't handle.
> -        */
> -       if (unlikely(unaligned_ctl && !misaligned_emu_detected)) {
> -               pr_crit("CPU misaligned accesses non homogeneous (expected all emulated)\n");
> -               while (true)
> -                       cpu_relax();
> -       }

This chunk was meant to detect and refuse to run on a system where a
heterogeneous CPU is hotplugged into a previously homogenous system.
The commit message doesn't mention this change, how come you
deleted it?


> -
> -       return misaligned_emu_detected;
>  }
>
>  bool check_unaligned_access_emulated_all_cpus(void)
> @@ -562,8 +548,11 @@ bool check_unaligned_access_emulated_all_cpus(void)
>          * accesses emulated since tasks requesting such control can run on any
>          * CPU.
>          */
> +       schedule_on_each_cpu(check_unaligned_access_emulated);
> +
>         for_each_online_cpu(cpu)
> -               if (!check_unaligned_access_emulated(cpu))
> +               if (per_cpu(misaligned_access_speed, cpu)
> +                   != RISCV_HWPROBE_MISALIGNED_EMULATED)
>                         return false;
>
>         unaligned_ctl = true;
> --
> 2.45.2
>