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Subject: Re: [RFC PATCH for 4.15 05/10] x86: Introduce sync_core_before_usermode
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <1007217384.13056.1510352430448.JavaMail.zimbra@efficios.com>
Date:   Fri, 10 Nov 2017 14:36:58 -0800
Cc:     Andy Lutomirski <luto@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Andrew Hunter <ahh@google.com>,
        maged michael <maged.michael@gmail.com>,
        Avi Kivity <avi@scylladb.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Dave Watson <davejwatson@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        "Russell King, ARM Linux" <linux@armlinux.org.uk>,
        Greg Hackmann <ghackmann@google.com>,
        Will Deacon <will.deacon@arm.com>,
        David Sehr <sehr@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        x86 <x86@kernel.org>, linux-arch <linux-arch@vger.kernel.org>
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Message-Id: <FC7466BE-FDEE-4FB8-ADA5-98C38050F7DF@amacapital.net>
References: <20171110213717.12457-1-mathieu.desnoyers@efficios.com> <20171110213717.12457-6-mathieu.desnoyers@efficios.com> <CALCETrWV+bgUPoS7NqVYhoi7hOyvsfoWw5CnyMrkYz=HYznmXQ@mail.gmail.com> <1007217384.13056.1510352430448.JavaMail.zimbra@efficios.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 10, 2017, at 2:20 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios=
.com> wrote:
>=20
> ----- On Nov 10, 2017, at 5:02 PM, Andy Lutomirski luto@kernel.org wrote:
>=20
>> 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.
>>>=20
>>> This is needed to fix an existing core serialization bug on
>>> thread migration, and also needed by the membarrier "sync_core" command.=

>>>=20
>>> Architectures defining the sync_core_before_usermode() static inline
>>> need to define ARCH_HAS_SYNC_CORE_BEFORE_USERMODE.
>>>=20
>>> 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(+)
>>>=20
>>> 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/pro=
cessor.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
>>> }
>>>=20
>>> +/*
>>> + * Ensure that a core serializing instruction is issued before returnin=
g
>>> + * to user-mode. x86 implements return to user-space through sysexit an=
d
>>> + * sysretq, which are not core serializing.
>>> + */
>>> +static inline void sync_core_before_usermode(void)
>>> +{
>>> +       sync_core();
>>> +}
>>=20
>> Make this if (!in_interrupt()) sync_core(); please.  We can optimize
>> it better later on.
>=20
> Sure, done. It will be part of the next version of that patch.
>=20
> Thanks!

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

>=20
> Mathieu
>=20
>=20
> --=20
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
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