diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 39e069b..bd31b25 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -426,3 +426,63 @@ For 32-bit we have the following conventions - kernel is built with
 .endm
 
 #endif /* CONFIG_SMP */
+
+#ifdef CONFIG_X86_64
+
+/* rdi:	arg1 ... normal C conventions. rax is saved/restored. */
+.macro THUNK name, func
+SYM_FUNC_START(\name)
+	pushq %rbp
+	movq %rsp, %rbp
+
+	pushq %rdi
+	pushq %rsi
+	pushq %rdx
+	pushq %rcx
+	pushq %rax
+	pushq %r8
+	pushq %r9
+	pushq %r10
+	pushq %r11
+
+	call \func
+
+	popq %r11
+	popq %r10
+	popq %r9
+	popq %r8
+	popq %rax
+	popq %rcx
+	popq %rdx
+	popq %rsi
+	popq %rdi
+	popq %rbp
+	RET
+SYM_FUNC_END(\name)
+	_ASM_NOKPROBE(\name)
+.endm
+
+#else /* CONFIG_X86_32 */
+
+/* put return address in eax (arg1) */
+.macro THUNK name, func, put_ret_addr_in_eax=0
+SYM_CODE_START_NOALIGN(\name)
+	pushl %eax
+	pushl %ecx
+	pushl %edx
+
+	.if \put_ret_addr_in_eax
+	/* Place EIP in the arg1 */
+	movl 3*4(%esp), %eax
+	.endif
+
+	call \func
+	popl %edx
+	popl %ecx
+	popl %eax
+	RET
+	_ASM_NOKPROBE(\name)
+SYM_CODE_END(\name)
+	.endm
+
+#endif
diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index 8c8d38f..582731f 100644
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -7,6 +7,8 @@
 #include <linux/linkage.h>
 #include <asm/msr-index.h>
 
+#include "calling.h"
+
 .pushsection .noinstr.text, "ax"
 
 SYM_FUNC_START(entry_ibpb)
@@ -20,3 +22,5 @@ SYM_FUNC_END(entry_ibpb)
 EXPORT_SYMBOL_GPL(entry_ibpb);
 
 .popsection
+
+THUNK warn_thunk_thunk, __warn_thunk
diff --git a/arch/x86/entry/thunk_32.S b/arch/x86/entry/thunk_32.S
index 0103e10..da37f42 100644
--- a/arch/x86/entry/thunk_32.S
+++ b/arch/x86/entry/thunk_32.S
@@ -4,33 +4,15 @@
  * Copyright 2008 by Steven Rostedt, Red Hat, Inc
  *  (inspired by Andi Kleen's thunk_64.S)
  */
-	#include <linux/export.h>
-	#include <linux/linkage.h>
-	#include <asm/asm.h>
 
-	/* put return address in eax (arg1) */
-	.macro THUNK name, func, put_ret_addr_in_eax=0
-SYM_CODE_START_NOALIGN(\name)
-	pushl %eax
-	pushl %ecx
-	pushl %edx
+#include <linux/export.h>
+#include <linux/linkage.h>
+#include <asm/asm.h>
 
-	.if \put_ret_addr_in_eax
-	/* Place EIP in the arg1 */
-	movl 3*4(%esp), %eax
-	.endif
+#include "calling.h"
 
-	call \func
-	popl %edx
-	popl %ecx
-	popl %eax
-	RET
-	_ASM_NOKPROBE(\name)
-SYM_CODE_END(\name)
-	.endm
-
-	THUNK preempt_schedule_thunk, preempt_schedule
-	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
-	EXPORT_SYMBOL(preempt_schedule_thunk)
-	EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
+THUNK preempt_schedule_thunk, preempt_schedule
+THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
+EXPORT_SYMBOL(preempt_schedule_thunk)
+EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
 
diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index 416b400..119ebdc 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -9,39 +9,6 @@
 #include "calling.h"
 #include <asm/asm.h>
 
-	/* rdi:	arg1 ... normal C conventions. rax is saved/restored. */
-	.macro THUNK name, func
-SYM_FUNC_START(\name)
-	pushq %rbp
-	movq %rsp, %rbp
-
-	pushq %rdi
-	pushq %rsi
-	pushq %rdx
-	pushq %rcx
-	pushq %rax
-	pushq %r8
-	pushq %r9
-	pushq %r10
-	pushq %r11
-
-	call \func
-
-	popq %r11
-	popq %r10
-	popq %r9
-	popq %r8
-	popq %rax
-	popq %rcx
-	popq %rdx
-	popq %rsi
-	popq %rdi
-	popq %rbp
-	RET
-SYM_FUNC_END(\name)
-	_ASM_NOKPROBE(\name)
-	.endm
-
 THUNK preempt_schedule_thunk, preempt_schedule
 THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
 EXPORT_SYMBOL(preempt_schedule_thunk)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 2c0679e..5575461 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -357,6 +357,8 @@ extern void entry_ibpb(void);
 
 extern void (*x86_return_thunk)(void);
 
+extern void __warn_thunk(void);
+
 #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING
 extern void call_depth_return_thunk(void);
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index f277541..a78892b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2850,3 +2850,8 @@ ssize_t cpu_show_gds(struct device *dev, struct device_attribute *attr, char *bu
 	return cpu_show_common(dev, attr, buf, X86_BUG_GDS);
 }
 #endif
+
+void __warn_thunk(void)
+{
+	WARN_ONCE(1, "Unpatched return thunk in use. This should not happen!\n");
+}
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 0045153..721b528 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -369,19 +369,16 @@ SYM_FUNC_END(call_depth_return_thunk)
  * 'JMP __x86_return_thunk' sites are changed to something else by
  * apply_returns().
  *
- * This should be converted eventually to call a warning function which
- * should scream loudly when the default return thunk is called after
- * alternatives have been applied.
- *
- * That warning function cannot BUG() because the bug splat cannot be
- * displayed in all possible configurations, leading to users not really
- * knowing why the machine froze.
+ * The ALTERNATIVE below adds a really loud warning to catch the case
+ * where the insufficient default return thunk ends up getting used for
+ * whatever reason like miscompilation or failure of
+ * objtool/alternatives/etc to patch all the return sites.
  */
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
-	ANNOTATE_UNRET_SAFE
-	ret
+	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
+		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
 	int3
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)
