[PATCH 3.16 033/410] x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP

[Ben Hutchings]

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wang Nan <wangnan0@xxxxxxxxxx>

commit b4d8327024637cb2a1f7910dcb5d0ad7a096f473 upstream.

Before this patch early_trap_init() installs DEBUG_STACK for
X86_TRAP_BP and X86_TRAP_DB. However, DEBUG_STACK doesn't work
correctly until cpu_init() <-- trap_init().

This patch passes 0 to set_intr_gate_ist() and
set_system_intr_gate_ist() instead of DEBUG_STACK to let it use
same stack as kernel, and installs DEBUG_STACK for them in
trap_init().

As core runs at ring 0 between early_trap_init() and
trap_init(), there is no chance to get a bad stack before
trap_init().

As NMI is also enabled in trap_init(), we don't need to care
about is_debug_stack() and related things used in
arch/x86/kernel/nmi.c.

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: <dave.hansen@xxxxxxxxxxxxxxx>
Cc: <lizefan@xxxxxxxxxx>
Cc: <luto@xxxxxxxxxxxxxx>
Cc: <oleg@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/1424929779-13174-1-git-send-email-wangnan0@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/traps.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -773,9 +773,17 @@ dotraplinkage void do_iret_error(struct
 /* Set of traps needed for early debugging. */
 void __init early_trap_init(void)
 {
-	set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
+	/*
+	 * Don't set ist to DEBUG_STACK as it doesn't work until TSS is
+	 * ready in cpu_init() <-- trap_init(). Before trap_init(), CPU
+	 * runs at ring 0 so it is impossible to hit an invalid stack.
+	 * Using the original stack works well enough at this early
+	 * stage. DEBUG_STACK will be equipped after cpu_init() in
+	 * trap_init().
+	 */
+	set_intr_gate_ist(X86_TRAP_DB, &debug, 0);
 	/* int3 can be called from all */
-	set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
+	set_system_intr_gate_ist(X86_TRAP_BP, &int3, 0);
 #ifdef CONFIG_X86_32
 	set_intr_gate(X86_TRAP_PF, page_fault);
 #endif
@@ -853,6 +861,15 @@ void __init trap_init(void)
 	 */
 	cpu_init();
 
+	/*
+	 * X86_TRAP_DB and X86_TRAP_BP have been set
+	 * in early_trap_init(). However, DEBUG_STACK works only after
+	 * cpu_init() loads TSS. See comments in early_trap_init().
+	 */
+	set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
+	/* int3 can be called from all */
+	set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
+
 	x86_init.irqs.trap_init();
 
 #ifdef CONFIG_X86_64