[RFC PATCH for 4.15 05/10] x86: Introduce sync_core_before_usermode

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

This is needed to fix an existing core serialization bug on
thread migration, and also needed by the membarrier "sync_core" command.

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

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
CC: Peter Zijlstra <redacted>
CC: Andy Lutomirski <redacted>
CC: Paul E. McKenney <redacted>
CC: Boqun Feng <redacted>
CC: Andrew Hunter <redacted>
CC: Maged Michael <redacted>
CC: Avi Kivity <avi-VrcmuVmyx1hWk0Htik3J/w@public.gmane.org>
CC: Benjamin Herrenschmidt <redacted>
CC: Paul Mackerras <redacted>
CC: Michael Ellerman <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>
CC: Dave Watson <redacted>
CC: Thomas Gleixner <redacted>
CC: Ingo Molnar <redacted>
CC: "H. Peter Anvin" <redacted>
CC: Andrea Parri <redacted>
CC: Russell King <redacted>
CC: Greg Hackmann <redacted>
CC: Will Deacon <redacted>
CC: David Sehr <redacted>
CC: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
CC: x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org
CC: linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
---
 arch/x86/Kconfig                 |  1 +
 arch/x86/include/asm/processor.h | 10 ++++++++++
 include/linux/processor.h        |  6 ++++++
 3 files changed, 17 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 01f78c1d40b5..54fbb8960d94 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig

@@ -62,6 +62,7 @@ config X86
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_HAS_STRICT_MODULE_RWX
+	select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_ZONE_DEVICE		if X86_64
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index bdac19ab2488..6ce996a7c730 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h

@@ -706,6 +706,16 @@ static inline void sync_core(void)
 #endif
 }
 
+/*
+ * Ensure that a core serializing instruction is issued before returning
+ * to user-mode. x86 implements return to user-space through sysexit and
+ * sysretq, which are not core serializing.
+ */
+static inline void sync_core_before_usermode(void)
+{
+	sync_core();
+}
+
 extern void select_idle_routine(const struct cpuinfo_x86 *c);
 extern void amd_e400_c1e_apic_setup(void);
 

diff --git a/include/linux/processor.h b/include/linux/processor.h
index dbc952eec869..7d12e6fa050e 100644
--- a/include/linux/processor.h
+++ b/include/linux/processor.h

@@ -68,4 +68,10 @@ do {								\
 
 #endif
 
+#ifndef ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
+static inline sync_core_before_usermode(void)
+{
+}
+#endif
+
 #endif /* _LINUX_PROCESSOR_H */

-- 
2.11.0