[PATCH for 4.16 v2 06/11] Introduce sync_core_before_usermode

[Mathieu Desnoyers]

Introduce an architecture function that ensures the current CPU
issues a core serializing instruction before returning to usermode.

This is needed for the membarrier "sync_core" command.

Architectures defining the sync_core_before_usermode() static inline
need to select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxx>
CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CC: Boqun Feng <boqun.feng@xxxxxxxxx>
CC: Andrew Hunter <ahh@xxxxxxxxxx>
CC: Maged Michael <maged.michael@xxxxxxxxx>
CC: Avi Kivity <avi@xxxxxxxxxxxx>
CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
CC: Paul Mackerras <paulus@xxxxxxxxx>
CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
CC: Dave Watson <davejwatson@xxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Andrea Parri <parri.andrea@xxxxxxxxx>
CC: Russell King <linux@xxxxxxxxxxxxxxx>
CC: Greg Hackmann <ghackmann@xxxxxxxxxx>
CC: Will Deacon <will.deacon@xxxxxxx>
CC: David Sehr <sehr@xxxxxxxxxx>
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Arnd Bergmann <arnd@xxxxxxxx>
CC: x86@xxxxxxxxxx
CC: linux-arch@xxxxxxxxxxxxxxx
---
Changes since v1:
- Introduce include/linux/sync_core.h
---
 include/linux/sync_core.h | 21 +++++++++++++++++++++
 init/Kconfig              |  3 +++
 2 files changed, 24 insertions(+)
 create mode 100644 include/linux/sync_core.h

diff --git a/include/linux/sync_core.h b/include/linux/sync_core.h
new file mode 100644
index 000000000000..013da4b8b327
--- /dev/null
+++ b/include/linux/sync_core.h
@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_SYNC_CORE_H
+#define _LINUX_SYNC_CORE_H
+
+#ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
+#include <asm/sync_core.h>
+#else
+/*
+ * This is a dummy sync_core_before_usermode() implementation that can be used
+ * on all architectures which return to user-space through core serializing
+ * instructions.
+ * If your architecture returns to user-space through non-core-serializing
+ * instructions, you need to write your own functions.
+ */
+static inline void sync_core_before_usermode(void)
+{
+}
+#endif
+
+#endif /* _LINUX_SYNC_CORE_H */
+
diff --git a/init/Kconfig b/init/Kconfig
index 2d118b6adee2..30208da2221f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1918,3 +1918,6 @@ config ASN1
 	  functions to call on what tags.
 
 source "kernel/Kconfig.locks"
+
+config ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
+	bool
-- 
2.11.0