Re: [External] Re: [PATCH v7 0/9] Parallel CPU bringup for x86_64

[Usama Arif <usama.arif@bytedance.com>]

On 09/02/2023 11:03, David Woodhouse wrote:
> On Thu, 2023-02-09 at 09:49 +0000, Usama Arif wrote:
>>
>> Its easy to test, just by doing
>> echo 0 > /sys/devices/system/cpu/cpu0/online;
>> echo 1 > /sys/devices/system/cpu/cpu0/online;
> 
> This one also fixes it for me. If we're happy with this approach, I'll
> work it into Thomas's original patch (and hopefully eventually he'll be
> happy enough with it and the commit message that he'll give us his
> Signed-off-by for it.)
> 

Yes, I think its better!

> 
> I could probably add a Co-developed-by: tglx for that first x2apic
> patch in the series too, but then it would *also* need his SoB and I
> didn't want to be owed two, so I just pasted his suggested code and
> didn't credit him.
> 
> 
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 5462464fe3ef..ea6052a97619 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -450,7 +450,16 @@ SYM_CODE_END(secondary_startup_64)
>   SYM_CODE_START(start_cpu0)
>          ANNOTATE_NOENDBR
>          UNWIND_HINT_EMPTY
> -       movq    initial_stack(%rip), %rsp
> +       /* Load the per-cpu base for CPU#0 */
> +       leaq    __per_cpu_offset(%rip), %rbx
> +       movq    (%rbx), %rbx
> +
> +       /* Find the idle task stack */
> +       movq    $idle_threads, %rcx
> +       addq    %rbx, %rcx
> +       movq    (%rcx), %rcx
> +       movq    TASK_threadsp(%rcx), %rsp
> +
>          jmp     .Ljump_to_C_code
>   SYM_CODE_END(start_cpu0)
>   #endif
> 
> I cut and pasted some of that, I'm not entirely sure why we have three
> instructions to do the equivalent of 'movq idle_threads(%ebx), %ecx'
> and may fix that in the original as I work this in.