[PATCH v3 5/5] x86/entry/64: Bypass enter_from_user_mode on non-context-tracking boots

Andy Lutomirski
2015-11-12T20:59:20Z

On CONFIG_CONTEXT_TRACKING kernels that have context tracking
disabled at runtime (which includes most distro kernels), we still
have the overhead of a call to enter_from_user_mode in interrupt and
exception entries.

If jump labels are available, this uses the jump label
infrastructure to skip the call.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/calling.h | 15 +++++++++++++++
arch/x86/entry/entry_64.S | 8 ++------
2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3c71dd947c7b..e32206e09868 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -1,3 +1,5 @@
+#include <linux/jump_label.h>
+
/*

x86 function call convention, 64-bit:
@@ -232,3 +234,16 @@ For 32-bit we have the following conventions - kernel is built with

#endif /* CONFIG_X86_64 */

+/*
+ * This does 'call enter_from_user_mode' unless we can avoid it based on
+ * kernel config or using the static jump infrastructure.
+ */
+.macro CALL_enter_from_user_mode
+#ifdef CONFIG_CONTEXT_TRACKING
+#ifdef HAVE_JUMP_LABEL
+ STATIC_JUMP_IF_FALSE .Lafter_call_\@, context_tracking_enabled, def=0
+#endif
+ call enter_from_user_mode
+.Lafter_call_\@:
+#endif
+.endm
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a55697d19824..9d34d3cfceb6 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -520,9 +520,7 @@ END(irq_entries_start)
*/
TRACE_IRQS_OFF

-#ifdef CONFIG_CONTEXT_TRACKING
- call enter_from_user_mode
-#endif
+ CALL_enter_from_user_mode

1:
/*
@@ -1066,9 +1064,7 @@ ENTRY(error_entry)
* (which can take locks).
*/
TRACE_IRQS_OFF
-#ifdef CONFIG_CONTEXT_TRACKING
- call enter_from_user_mode
-#endif
+ CALL_enter_from_user_mode
ret

.Lerror_entry_done:
--
2.5.0