start_kernel()
|-- setup_arch()
|   |-- parse_cmdline_early();  // SMP looks for "noht" and "acpismp=force"
|   |   `-- /* "noht" disables HyperThreading (2 logical cpus per Xeon) */
|   |       if (!memcmp(from, "noht", 4)) {
|   |           disable_x86_ht = 1;
|   |           set_bit(X86_FEATURE_HT, disabled_x86_caps);
|   |       }
|   |       /* "acpismp=force" forces parsing and use of the ACPI SMP table */
|   |       else if (!memcmp(from, "acpismp=force", 13))
|   |           enable_acpi_smp_table = 1;
|   |-- setup_memory();         // reserve memory for MP configuration table
|   |   |-- reserve_bootmem(PAGE_SIZE, PAGE_SIZE);
|   |   `-- find_smp_config();
|   |       `-- find_intel_smp();
|   |           `-- smp_scan_config();
|   |               |-- set flag smp_found_config
|   |               |-- set MP floating pointer mpf_found
|   |               `-- reserve_bootmem(mpf_found, PAGE_SIZE);
|   |-- if (disable_x86_ht) {   // if HyperThreading feature disabled
|   |       clear_bit(X86_FEATURE_HT, &boot_cpu_data.x86_capability[0]);
|   |       set_bit(X86_FEATURE_HT, disabled_x86_caps);
|   |       enable_acpi_smp_table = 0;
|   |   }
|   |-- if (test_bit(X86_FEATURE_HT, &boot_cpu_data.x86_capability[0]))
|   |       enable_acpi_smp_table = 1;
|   |-- smp_alloc_memory();
|   |   `-- /* reserve AP processor's real-mode code space in low memory */
|   |       trampoline_base = (void *) alloc_bootmem_low_pages(PAGE_SIZE);
|   `-- get_smp_config();     /* get boot-time MP configuration */
|       |-- config_acpi_tables();
|       |   |-- memset(&acpi_boot_ops, 0, sizeof(acpi_boot_ops));
|       |   |-- acpi_boot_ops[ACPI_APIC] = acpi_parse_madt;
|       |   `-- /* Set have_acpi_tables to indicate using
|       |        * MADT in the ACPI tables; Use MPS tables if failed. */
|       |       if (enable_acpi_smp_table && !acpi_tables_init())
|       |           have_acpi_tables = 1;
|       |-- set pic_mode
|       |   /* =1, if the IMCR is present and PIC Mode is implemented;
|       |    * =0, otherwise Virtual Wire Mode is implemented. */
|       |-- save local APIC address in mp_lapic_addr
|       `-- scan for MP configuration table entries, like
|             MP_PROCESSOR, MP_BUS, MP_IOAPIC, MP_INTSRC and MP_LINTSRC.
|-- trap_init();
|   `-- init_apic_mappings();   // setup PTE for APIC
|       |-- /* If no local APIC can be found then set up a fake all
|       |    * zeroes page to simulate the local APIC and another
|       |    * one for the IO-APIC. */
|       |   if (!smp_found_config && detect_init_APIC()) {
|       |       apic_phys = (unsigned long) alloc_bootmem_pages(PAGE_SIZE);
|       |       apic_phys = __pa(apic_phys);
|       |   } else
|       |       apic_phys = mp_lapic_addr;
|       |-- /* map local APIC address,
|       |    *   mp_lapic_addr (0xfee00000) in most case,
|       |    *   to linear address FIXADDR_TOP (0xffffe000) */
|       |   set_fixmap_nocache(FIX_APIC_BASE, apic_phys);
|       |-- /* Fetch the APIC ID of the BSP in case we have a
|       |    * default configuration (or the MP table is broken). */
|       |   if (boot_cpu_physical_apicid == -1U)
|       |       boot_cpu_physical_apicid = GET_APIC_ID(apic_read(APIC_ID));
|       `-- // map IOAPIC address to uncacheable linear address
|           set_fixmap_nocache(idx, ioapic_phys);
|       // Now we can use linear address to access APIC space.
|-- init_IRQ();
|   |-- init_ISA_irqs();
|   |   |-- /* An initial setup of the virtual wire mode. */
|   |   |   init_bsp_APIC();
|   |   `-- init_8259A(auto_eoi=0);
|   `-- setup SMP/APIC interrupt handlers, esp. IPI.
`-- mem_init();
    `-- /* delay zapping low mapping entries for SMP: zap_low_mappings() */