[src/trunk]: src/sys/arch/amd64/amd64 Explain a little.

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/98f708f3c918
branches:  trunk
changeset: 346934:98f708f3c918
user:      maxv <maxv%NetBSD.org@localhost>
date:      Sun Aug 07 09:04:55 2016 +0000

description:
Explain a little.

diffstat:

 sys/arch/amd64/amd64/amd64_trap.S |  31 ++++++++++++++++++++-----------
 1 files changed, 20 insertions(+), 11 deletions(-)

diffs (73 lines):

diff -r 285274975fe6 -r 98f708f3c918 sys/arch/amd64/amd64/amd64_trap.S
--- a/sys/arch/amd64/amd64/amd64_trap.S Sun Aug 07 07:13:57 2016 +0000
+++ b/sys/arch/amd64/amd64/amd64_trap.S Sun Aug 07 09:04:55 2016 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: amd64_trap.S,v 1.3 2015/11/22 13:41:24 maxv Exp $      */
+/*     $NetBSD: amd64_trap.S,v 1.4 2016/08/07 09:04:55 maxv Exp $      */
 
 /*-
  * Copyright (c) 1998, 2007, 2008 The NetBSD Foundation, Inc.
@@ -66,7 +66,7 @@
 
 #if 0
 #include <machine/asm.h>
-__KERNEL_RCSID(0, "$NetBSD: amd64_trap.S,v 1.3 2015/11/22 13:41:24 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: amd64_trap.S,v 1.4 2016/08/07 09:04:55 maxv Exp $");
 #endif
 
 /*
@@ -229,28 +229,36 @@
 IDTVEC(trap0b)         /* #NP() Segment not present */
        TRAP_NJ(T_SEGNPFLT)
        jmp     check_swapgs
-IDTVEC_END(trap0b)             /* #NP() Segment not present */
+IDTVEC_END(trap0b)
 
 IDTVEC(trap0c)         /* #SS() Stack exception */
        TRAP_NJ(T_STKFLT)
        jmp     check_swapgs
-IDTVEC_END(trap0c)             /* #SS() Stack exception */
+IDTVEC_END(trap0c)
 
 IDTVEC(trap0d)         /* #GP() General protection */
        TRAP_NJ(T_PROTFLT)
 #ifdef check_swapgs
        jmp     check_swapgs
 #else
-/* We need to worry about traps while the kernel %gs_base isn't loaded.
- * These are either loads to %gs (only 32bit) or faults on iret during
- * return to user. */
+/*
+ * We need to worry about traps in kernel mode while the kernel %gs isn't
+ * loaded. These are either faults on iretq during return to user or loads to
+ * %gs.
+ *
+ * When such traps happen, we have CPL=0 and %gs=userland, and we must perform
+ * an additional swapgs to get %gs=kernel.
+ */
 check_swapgs:
        INTRENTRY_L(3f,1:)
-2:     sti
+2:
+       sti
        jmp     calltrap
 3:
-       /* Trap in kernel mode. */
-       /* If faulting instruction is 'iret' we may need to do a 'swapgs'. */
+       /*
+        * Trap in kernel mode.
+        */
+       /* Case 1: fault on iretq? */
        movq    TF_RIP(%rsp),%rax
        cmpw    $0xcf48,(%rax)          /* Faulting instruction is iretq ? */
        jne     5f                      /* Jump if not */
@@ -259,7 +267,8 @@
        je      2b                      /* jump if iret was to kernel  */
        jmp     1b                      /* to user - must restore %gs */
 5:
-       /* Not 'iret', all moves to %gs also need a swapgs */
+
+       /* Case 2: move to %gs? */
        movw    (%rax),%ax
        andb    $070,%ah                /* mask mod/rm from mod/reg/rm */
        cmpw    $0x8e+050*256,%ax       /* Any move to %gs (reg 5) */