v4.19.13 snapshot.
diff --git a/tools/testing/selftests/x86/thunks.S b/tools/testing/selftests/x86/thunks.S
new file mode 100644
index 0000000..ce8a995
--- /dev/null
+++ b/tools/testing/selftests/x86/thunks.S
@@ -0,0 +1,67 @@
+/*
+ * thunks.S - assembly helpers for mixed-bitness code
+ * Copyright (c) 2015 Andrew Lutomirski
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * These are little helpers that make it easier to switch bitness on
+ * the fly.
+ */
+
+ .text
+
+ .global call32_from_64
+ .type call32_from_64, @function
+call32_from_64:
+ // rdi: stack to use
+ // esi: function to call
+
+ // Save registers
+ pushq %rbx
+ pushq %rbp
+ pushq %r12
+ pushq %r13
+ pushq %r14
+ pushq %r15
+ pushfq
+
+ // Switch stacks
+ mov %rsp,(%rdi)
+ mov %rdi,%rsp
+
+ // Switch to compatibility mode
+ pushq $0x23 /* USER32_CS */
+ pushq $1f
+ lretq
+
+1:
+ .code32
+ // Call the function
+ call *%esi
+ // Switch back to long mode
+ jmp $0x33,$1f
+ .code64
+
+1:
+ // Restore the stack
+ mov (%rsp),%rsp
+
+ // Restore registers
+ popfq
+ popq %r15
+ popq %r14
+ popq %r13
+ popq %r12
+ popq %rbp
+ popq %rbx
+
+ ret
+
+.size call32_from_64, .-call32_from_64