From d2ca2ffb15f04aab7210757d7c7dabb33ab720d0 Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Tue, 5 May 2015 20:04:15 +0200
Subject: [PATCH gnumach 07/10] document

---
 i386/i386/locore.S | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/i386/i386/locore.S b/i386/i386/locore.S
index 99fbfb6..4c2a253 100644
--- a/i386/i386/locore.S
+++ b/i386/i386/locore.S
@@ -1184,6 +1184,18 @@ syscall_addr:
 					/* set error code - read user space */
 	jmp	_take_trap		/* treat as a trap */
 
+/*
+ * SYSENTER-based system calls.
+ *
+ * Calling convention:
+ *   %eax - syscall number
+ *   %ebx - syscall argument 1
+ *   %ecx - syscall argument 2
+ *   %edx - syscall argument 3
+ *   %esi - syscall argument 4
+ *   %edi - userspace return address
+ *   %ebp - userspace stack pointer
+ */
 
 /* User stub for calling the kernel using the sysenter instruction.  */
 	.globl user_sysenter_stub_start
@@ -1191,9 +1203,9 @@ user_sysenter_stub_start:
 	push %ebp
 	mov %esp, %ebp
 	pushf
-	push %ebx
+	push %ebx		/* Store callee-saved registers.  */
 	push %esi
-	push %edi		/* xxxmore callee-saved registers?  */
+	push %edi
 	mov 8(%ebp), %eax	/* Move syscall number into %eax.  */
 	mov 12(%ebp), %ebx	/* Move first argument into %ebx.  */
 	mov 16(%ebp), %ecx	/* Move second argument into %ecx.  */
@@ -1216,14 +1228,9 @@ user_sysenter_stub_end:
 /*
  * SYSENTER entry point.
  *
- * Calling convention:
- *   %eax - syscall number
- *   %ebx - syscall argument 1
- *   %ecx - syscall argument 2
- *   %edx - syscall argument 3
- *   %esi - syscall argument 4
- *   %edi - userspace return address
- *   %ebp - userspace stack pointer
+ * Control enters at `sysenter_entry' with %esp pointing to the
+ * per-cpu sysenter stack.  We store all arguments here.  We keep %ebp
+ * pointing to the top of this structure to copy the arguments.
  *
  * sysenter stack layout:
  *
-- 
2.1.4