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

----- On Nov 10, 2017, at 5:02 PM, Andy Lutomirski luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org wrote:

On Fri, Nov 10, 2017 at 1:37 PM, Mathieu Desnoyers
[off-list ref] wrote:

quoted

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>
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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();
+}

Make this if (!in_interrupt()) sync_core(); please.  We can optimize
it better later on.

Sure, done. It will be part of the next version of that patch.

Thanks!

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com