--- zzzz-none-000/linux-3.10.107/arch/x86/include/uapi/asm/ptrace.h	2017-06-27 09:49:32.000000000 +0000
+++ scorpion-7490-727/linux-3.10.107/arch/x86/include/uapi/asm/ptrace.h	2021-02-04 17:41:59.000000000 +0000
@@ -41,13 +41,17 @@
 #ifndef __KERNEL__
 
 struct pt_regs {
+/*
+ * C ABI says these regs are callee-preserved. They aren't saved on kernel entry
+ * unless syscall needs a complete, fully filled "struct pt_regs".
+ */
 	unsigned long r15;
 	unsigned long r14;
 	unsigned long r13;
 	unsigned long r12;
 	unsigned long rbp;
 	unsigned long rbx;
-/* arguments: non interrupts/non tracing syscalls only save up to here*/
+/* These regs are callee-clobbered. Always saved on kernel entry. */
 	unsigned long r11;
 	unsigned long r10;
 	unsigned long r9;
@@ -57,9 +61,12 @@
 	unsigned long rdx;
 	unsigned long rsi;
 	unsigned long rdi;
+/*
+ * On syscall entry, this is syscall#. On CPU exception, this is error code.
+ * On hw interrupt, it's IRQ number:
+ */
 	unsigned long orig_rax;
-/* end of arguments */
-/* cpu exception frame or undefined */
+/* Return frame for iretq */
 	unsigned long rip;
 	unsigned long cs;
 	unsigned long eflags;