diff --git a/ostd/src/arch/x86/boot/smp.rs b/ostd/src/arch/x86/boot/smp.rs
index 450c3efd..a6dae25d 100644
--- a/ostd/src/arch/x86/boot/smp.rs
+++ b/ostd/src/arch/x86/boot/smp.rs
@@ -27,6 +27,8 @@
 //! This sequence does not need to be strictly followed, and there may be
 //! different considerations in different systems.
 
+use acpi::madt::MadtEntry;
+
 use crate::{
     arch::x86::kernel::{
         acpi::get_acpi_tables,
@@ -45,19 +47,54 @@ use crate::{
 /// This function needs to be called after the OS initializes the ACPI table.
 pub(crate) fn get_num_processors() -> Option<u32> {
     let acpi_tables = get_acpi_tables()?;
-    let mut local_apic_counts = 0;
-    acpi_tables
-        .find_table::<acpi::madt::Madt>()
-        .unwrap()
+    let madt_table = acpi_tables.find_table::<acpi::madt::Madt>().ok()?;
+
+    // In the UEFI spec [1], for compatibility, the firmware will provide
+    // "Processor X2APIC structure" for local APIC ID values >= 255 and
+    // "Processor APIC structure" for local APIC ID values < 255, even if it is
+    // in the x2APIC mode.
+    // [1]: https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/05_ACPI_Software_Programming_Model/ACPI_Software_Programming_Model.html#processor-local-x2apic-structure
+    let local_apic_counts = madt_table
         .get()
         .entries()
-        .for_each(|entry| {
-            if let acpi::madt::MadtEntry::LocalApic(_) = entry {
-                local_apic_counts += 1;
+        .filter(|e| match e {
+            MadtEntry::LocalX2Apic(entry) => {
+                let processor_uid = entry.processor_uid;
+                let x2apic_id = entry.x2apic_id;
+                log::trace!(
+                    "Found a local x2APIC entry in MADT: processor UID = {}, x2APIC ID = {}",
+                    processor_uid,
+                    x2apic_id,
+                );
+                true
             }
-        });
+            MadtEntry::LocalApic(entry) => {
+                let processor_id = entry.processor_id;
+                let apic_id = entry.apic_id as u32;
+                log::trace!(
+                    "Found a local APIC entry in MADT: processor ID = {}, APIC ID = {}",
+                    processor_id,
+                    apic_id,
+                );
+                // Check if APIC entries also show up as x2APIC entries.
+                if madt_table
+                    .get()
+                    .entries()
+                    .any(|e| matches!(e, MadtEntry::LocalX2Apic(e) if e.x2apic_id == apic_id))
+                {
+                    log::warn!(
+                        "Firmware bug: in MADT, APIC ID {} is also listed as a x2APIC ID",
+                        apic_id
+                    );
+                    return false;
+                }
+                true
+            }
+            _ => false,
+        })
+        .count();
 
-    Some(local_apic_counts)
+    Some(local_apic_counts as u32)
 }
 
 /// Brings up all application processors.
diff --git a/ostd/src/boot/smp.rs b/ostd/src/boot/smp.rs
index 37a1b304..2f16a392 100644
--- a/ostd/src/boot/smp.rs
+++ b/ostd/src/boot/smp.rs
@@ -8,8 +8,8 @@ use core::sync::atomic::{AtomicBool, Ordering};
 use spin::Once;
 
 use crate::{
-    arch::boot::smp::{bringup_all_aps, get_num_processors},
-    cpu,
+    arch::boot::smp::bringup_all_aps,
+    cpu::{self, num_cpus},
     mm::{
         frame::{meta::KernelMeta, Segment},
         paddr_to_vaddr, FrameAllocOptions, PAGE_SIZE,
@@ -49,12 +49,11 @@ static AP_LATE_ENTRY: Once<fn()> = Once::new();
 /// However, the function need to be called before any `cpu_local!` variables are
 /// accessed, including the APIC instance.
 pub fn boot_all_aps() {
-    // TODO: support boot protocols without ACPI tables, e.g., Multiboot
-    let Some(num_cpus) = get_num_processors() else {
-        log::warn!("No processor information found. The kernel operates with a single processor.");
+    let num_cpus = num_cpus() as u32;
+    if num_cpus == 1 {
         return;
-    };
-    log::info!("Found {} processors.", num_cpus);
+    }
+    log::info!("Booting {} processors.", num_cpus - 1);
 
     // We currently assumes that bootstrap processor (BSP) have always the
     // processor ID 0. And the processor ID starts from 0 to `num_cpus - 1`.