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@efficios.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Andy Lutomirski <luto@kernel.org>
CC: Paul E. McKenney <redacted>
CC: Boqun Feng <redacted>
CC: Andrew Hunter <redacted>
CC: Maged Michael <redacted>
CC: Avi Kivity <redacted>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <redacted>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Dave Watson <redacted>
CC: Thomas Gleixner <redacted>
CC: Ingo Molnar <mingo@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andrea Parri <parri.andrea@gmail.com>
CC: Russell King <linux@armlinux.org.uk>
CC: Greg Hackmann <redacted>
CC: Will Deacon <redacted>
CC: David Sehr <redacted>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: x86@kernel.org
CC: linux-arch@vger.kernel.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();
+}