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

[Mathieu Desnoyers]

----- On Nov 10, 2017, at 5:02 PM, Andy Lutomirski luto@xxxxxxxxxx wrote:

> On Fri, Nov 10, 2017 at 1:37 PM, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> CC: Andy Lutomirski <luto@xxxxxxxxxx>
>> CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>> CC: Boqun Feng <boqun.feng@xxxxxxxxx>
>> CC: Andrew Hunter <ahh@xxxxxxxxxx>
>> CC: Maged Michael <maged.michael@xxxxxxxxx>
>> CC: Avi Kivity <avi@xxxxxxxxxxxx>
>> CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
>> CC: Paul Mackerras <paulus@xxxxxxxxx>
>> CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> CC: Dave Watson <davejwatson@xxxxxx>
>> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> CC: Ingo Molnar <mingo@xxxxxxxxxx>
>> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> CC: Andrea Parri <parri.andrea@xxxxxxxxx>
>> CC: Russell King <linux@xxxxxxxxxxxxxxx>
>> CC: Greg Hackmann <ghackmann@xxxxxxxxxx>
>> CC: Will Deacon <will.deacon@xxxxxxx>
>> CC: David Sehr <sehr@xxxxxxxxxx>
>> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> CC: x86@xxxxxxxxxx
>> CC: linux-arch@xxxxxxxxxxxxxxx
>> ---
>>  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