Module Name: src
Committed By: maxv
Date: Sun Aug 7 09:04:55 UTC 2016
Modified Files:
src/sys/arch/amd64/amd64: amd64_trap.S
Log Message:
Explain a little.
To generate a diff of this commit:
cvs rdiff -u -r1.3 -r1.4 src/sys/arch/amd64/amd64/amd64_trap.S
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/amd64/amd64/amd64_trap.S
diff -u src/sys/arch/amd64/amd64/amd64_trap.S:1.3 src/sys/arch/amd64/amd64/amd64_trap.S:1.4
--- src/sys/arch/amd64/amd64/amd64_trap.S:1.3 Sun Nov 22 13:41:24 2015
+++ src/sys/arch/amd64/amd64/amd64_trap.S Sun Aug 7 09:04:55 2016
@@ -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_END(trap0a)
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 @@ check_swapgs:
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) */
