diff --git a/kernel/src/cpu/x86/apic.rs b/kernel/src/cpu/x86/apic.rs
new file mode 100644
index 000000000..68156629d
--- /dev/null
+++ b/kernel/src/cpu/x86/apic.rs
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: MIT
+//
+// Copyright (c) Microsoft Corporation
+//
+// Author: Jon Lange <jlange@microsoft.com>
+
+pub const APIC_MSR_EOI: u32 = 0x80B;
+pub const APIC_MSR_ISR: u32 = 0x810;
+pub const APIC_MSR_ICR: u32 = 0x830;
+
+// Returns the MSR offset and bitmask to identify a specific vector in an
+// APIC register (IRR, ISR, or TMR).
+pub fn apic_register_bit(vector: usize) -> (u32, u32) {
+    let index: u8 = (vector & 0xFF) as u8;
+    ((index >> 5) as u32, 1 << (index & 0x1F))
+}
diff --git a/kernel/src/cpu/x86/mod.rs b/kernel/src/cpu/x86/mod.rs
index 7f7ba665a..d888b5830 100644
--- a/kernel/src/cpu/x86/mod.rs
+++ b/kernel/src/cpu/x86/mod.rs
@@ -4,4 +4,5 @@
 //
 // Authors: Thomas Leroy <tleroy@suse.de>
 
+pub mod apic;
 pub mod smap;
diff --git a/kernel/src/platform/native.rs b/kernel/src/platform/native.rs
index 003196a76..99125ee38 100644
--- a/kernel/src/platform/native.rs
+++ b/kernel/src/platform/native.rs
@@ -12,6 +12,7 @@ use crate::cpu::cpuid::CpuidResult;
 use crate::cpu::msr::{read_msr, write_msr};
 use crate::cpu::percpu::PerCpu;
 use crate::cpu::smp::create_ap_start_context;
+use crate::cpu::x86::apic::{APIC_MSR_EOI, APIC_MSR_ICR};
 use crate::error::SvsmError;
 use crate::hyperv;
 use crate::hyperv::{hyperv_setup_hypercalls, hyperv_start_cpu, is_hyperv_hypervisor};
@@ -33,9 +34,6 @@ use bootlib::platform::SvsmPlatformType;
 const MSR_APIC_BASE: u32 = 0x1B;
 const APIC_X2_ENABLE_MASK: u64 = 0xC00;
 
-const APIC_MSR_EOI: u32 = 0x80B;
-const APIC_MSR_ICR: u32 = 0x830;
-
 #[derive(Clone, Copy, Debug)]
 pub struct NativePlatform {
     is_hyperv: bool,
diff --git a/kernel/src/platform/tdp.rs b/kernel/src/platform/tdp.rs
index c14710762..e4d428aff 100644
--- a/kernel/src/platform/tdp.rs
+++ b/kernel/src/platform/tdp.rs
@@ -7,7 +7,9 @@
 use crate::address::{Address, PhysAddr, VirtAddr};
 use crate::console::init_svsm_console;
 use crate::cpu::cpuid::CpuidResult;
+use crate::cpu::msr::read_msr;
 use crate::cpu::percpu::PerCpu;
+use crate::cpu::x86::apic::{apic_register_bit, APIC_MSR_ISR};
 use crate::error::SvsmError;
 use crate::hyperv;
 use crate::io::IOPort;
@@ -171,11 +173,11 @@ impl SvsmPlatform for TdpPlatform {
 
     fn eoi(&self) {}
 
-    fn is_external_interrupt(&self, _vector: usize) -> bool {
+    fn is_external_interrupt(&self, vector: usize) -> bool {
         // Examine the APIC ISR to determine whether this interrupt vector is
         // active.  If so, it is assumed to be an external interrupt.
-        // TODO - add code to read the APIC ISR.
-        todo!();
+        let (msr, mask) = apic_register_bit(vector);
+        (read_msr(APIC_MSR_ISR + msr) & mask as u64) != 0
     }
 
     fn start_cpu(