linux-kernel - Re: Question about prefill_possible

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Date:   Wed, 8 Apr 2020 20:07:16 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: Question about prefill_possible_map

Hi Thomas,

  Seems you helped improve 2a51fe083eba7f, could you help with this question?

-- 
Sincerely,
Cao jin

On 3/27/20 2:38 PM, Cao jin wrote:
> Hi,
> 
> At the beginning of this function, there is:
> 
>     /* No boot processor was found in mptable or ACPI MADT */
>     if (!num_processors) {
>         if (boot_cpu_has(X86_FEATURE_APIC)) {
>             int apicid = boot_cpu_physical_apicid;
>             int cpu = hard_smp_processor_id();
> 
>             pr_warn("Boot CPU (id %d) not listed by BIOS\n", cpu);
> 
>             /* Make sure boot cpu is enumerated */
>             if (apic->cpu_present_to_apicid(0) == BAD_APICID &&
>                 apic->apic_id_valid(apicid))
>                 generic_processor_info(apicid, boot_cpu_apic_version);
>         }
> 
>         if (!num_processors)
>             num_processors = 1;
>     }
> 
> I see 3 cases will go through this code chunk,
> 
> 1. UP, has no APCI MADT & MP table, but has APIC
> 2. 2a51fe083eba7f: kdump kernel running on the hot-plugged CPU
> 3. normal UP(No table, no APIC)
> 
> And "if (boot_cpu_has(X86_FEATURE_APIC))" covers case 1 & 2 to my
> understanding.
> 
> But both cases 1 & 2 already have boot_cpu_physical_apicid initialized
> from APIC ID register:
> 1. init_apic_mappings --> register_lapic_address
> 2. early_acpi_boot_init --> early_acpi_process_madt -->...-->
>    register_lapic_address
> 
> So my question is: is variable "cpu" redundant? Could it be:
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 69881b2d446c..99fc03511568 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1456,9 +1456,9 @@ __init void prefill_possible_map(void)
>         if (!num_processors) {
>                 if (boot_cpu_has(X86_FEATURE_APIC)) {
>                         int apicid = boot_cpu_physical_apicid;
> -                       int cpu = hard_smp_processor_id();
> 
> -                       pr_warn("Boot CPU (id %d) not listed by BIOS\n", cpu);
> +                       pr_warn("Boot CPU (id %d) not listed by BIOS\n", \
> +                                boot_cpu_physical_apicid);
> 
>                         /* Make sure boot cpu is enumerated */
>                         if (apic->cpu_present_to_apicid(0) == BAD_APICID &&
>