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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Nov 10, 2017, at 5:20 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.

Of course, using in_interrupt() from linux/interrupt.h from
asm/processor.h is not such a good idea (circular dependency).

Any recommendation on where to move that static inline on x86 ?

Thanks,

Mathieu


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

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