diff --git a/qemu/target/arm/cpu.h b/qemu/target/arm/cpu.h
index 82ae969d..c633e406 100644
--- a/qemu/target/arm/cpu.h
+++ b/qemu/target/arm/cpu.h
@@ -820,6 +820,11 @@ struct ARMCPU {
      * prefix means a constant register.
      * Some of these registers are split out into a substructure that
      * is shared with the translators to control the ISA.
+     *
+     * Note that if you add an ID register to the ARMISARegisters struct
+     * you need to also update the 32-bit and 64-bit versions of the
+     * kvm_arm_get_host_cpu_features() function to correctly populate the
+     * field by reading the value from the KVM vCPU.
      */
     struct ARMISARegisters {
         uint32_t id_isar0;