linux-kernel - Question about prefill_possible

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Date:   Fri, 27 Mar 2020 14:38:23 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Question about prefill_possible_map

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 &&

-- 
Sincerely,
Cao jin