diff --git a/chips/qemu_arm_mps2/src/vectors_m3.rs b/chips/qemu_arm_mps2/src/vectors_m3.rs
index cc6afdcae4..f05cb384a0 100644
--- a/chips/qemu_arm_mps2/src/vectors_m3.rs
+++ b/chips/qemu_arm_mps2/src/vectors_m3.rs
@@ -13,6 +13,10 @@ use cortexm3::{CortexM3, CortexMVariant, initialize_ram_jump_to_main, unhandled_
 extern "C" {
     // _estack is not really a function, but it makes the types work.
     // You should never actually invoke it!!
+    //
+    // Keep this in the chip crate: `process_console` declares `_estack` as
+    // data, and LTO can't hold both types in one unit -- it renames ours to
+    // `_estack.1` and the link then fails, blaming an unrelated line.
     fn _estack();
 }
 
diff --git a/chips/qemu_arm_mps2/src/vectors_m4.rs b/chips/qemu_arm_mps2/src/vectors_m4.rs
index b18e2b6c26..e48bbd448c 100644
--- a/chips/qemu_arm_mps2/src/vectors_m4.rs
+++ b/chips/qemu_arm_mps2/src/vectors_m4.rs
@@ -13,6 +13,10 @@ use cortexm4::{CortexM4, CortexMVariant, initialize_ram_jump_to_main, unhandled_
 extern "C" {
     // _estack is not really a function, but it makes the types work.
     // You should never actually invoke it!!
+    //
+    // Keep this in the chip crate: `process_console` declares `_estack` as
+    // data, and LTO can't hold both types in one unit -- it renames ours to
+    // `_estack.1` and the link then fails, blaming an unrelated line.
     fn _estack();
 }