linux-kernel - Re: [RFC PATCH for 4.15 05/10] x86: Introduce sync_core_before

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Date:   Fri, 10 Nov 2017 22:20:30 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Greg Hackmann <ghackmann@...gle.com>,
        Will Deacon <will.deacon@....com>,
        David Sehr <sehr@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        x86 <x86@...nel.org>, linux-arch <linux-arch@...r.kernel.org>
Subject: 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@...nel.org wrote:

> On Fri, Nov 10, 2017 at 1:37 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.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@...icios.com>
>> CC: Peter Zijlstra <peterz@...radead.org>
>> CC: Andy Lutomirski <luto@...nel.org>
>> CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> CC: Boqun Feng <boqun.feng@...il.com>
>> CC: Andrew Hunter <ahh@...gle.com>
>> CC: Maged Michael <maged.michael@...il.com>
>> CC: Avi Kivity <avi@...lladb.com>
>> CC: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>> CC: Paul Mackerras <paulus@...ba.org>
>> CC: Michael Ellerman <mpe@...erman.id.au>
>> CC: Dave Watson <davejwatson@...com>
>> CC: Thomas Gleixner <tglx@...utronix.de>
>> CC: Ingo Molnar <mingo@...hat.com>
>> CC: "H. Peter Anvin" <hpa@...or.com>
>> CC: Andrea Parri <parri.andrea@...il.com>
>> CC: Russell King <linux@...linux.org.uk>
>> CC: Greg Hackmann <ghackmann@...gle.com>
>> CC: Will Deacon <will.deacon@....com>
>> CC: David Sehr <sehr@...gle.com>
>> CC: Linus Torvalds <torvalds@...ux-foundation.org>
>> CC: x86@...nel.org
>> CC: linux-arch@...r.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!

Mathieu


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