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    SubjectRe: [RFC PATCH for 4.15 05/10] x86: Introduce sync_core_before_usermode
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    > On Nov 10, 2017, at 2:20 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
    >
    > ----- On Nov 10, 2017, at 5:02 PM, Andy Lutomirski luto@kernel.org wrote:
    >
    >> On Fri, Nov 10, 2017 at 1:37 PM, Mathieu Desnoyers
    >> <mathieu.desnoyers@efficios.com> wrote:
    >>> 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 <paulmck@linux.vnet.ibm.com>
    >>> CC: Boqun Feng <boqun.feng@gmail.com>
    >>> CC: Andrew Hunter <ahh@google.com>
    >>> CC: Maged Michael <maged.michael@gmail.com>
    >>> CC: Avi Kivity <avi@scylladb.com>
    >>> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    >>> CC: Paul Mackerras <paulus@samba.org>
    >>> CC: Michael Ellerman <mpe@ellerman.id.au>
    >>> CC: Dave Watson <davejwatson@fb.com>
    >>> CC: Thomas Gleixner <tglx@linutronix.de>
    >>> 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 <ghackmann@google.com>
    >>> CC: Will Deacon <will.deacon@arm.com>
    >>> CC: David Sehr <sehr@google.com>
    >>> 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();
    >>> +}
    >>
    >> 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!

    Hmm. Can you also cc stable and make sure it applies by itself?

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