Re: [PATCH for 4.16 07/11] x86: Implement sync_core_before_usermode (v3)

[Mathieu Desnoyers]

----- On Jan 17, 2018, at 12:53 PM, Andy Lutomirski luto@xxxxxxxxxx wrote:

> On Wed, Jan 17, 2018 at 8:54 AM, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> Ensure that a core serializing instruction is issued before returning to
>> user-mode. x86 implements return to user-space through sysexit, sysrel,
>> and sysretq, which are not core serializing.
>>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> 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: 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
>> ---
>> Changes since v1:
>> - Fix prototype of sync_core_before_usermode in generic code (missing
>>   return type).
>> - Add linux/processor.h include to sched/core.c.
>> - Add ARCH_HAS_SYNC_CORE_BEFORE_USERMODE to init/Kconfig.
>> - Fix linux/processor.h ifdef to target
>>   CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE rather than
>>   ARCH_HAS_SYNC_CORE_BEFORE_USERMODE.
>> - Move empty static inline in processor.h to generic patch.
>> ---
>>  arch/x86/Kconfig                 |  1 +
>>  arch/x86/include/asm/processor.h | 10 ++++++++++
>>  2 files changed, 11 insertions(+)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 20da391b5f32..0b44c8dd0e95 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -61,6 +61,7 @@ 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 d3a67fba200a..3257d34dbb40 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -722,6 +722,16 @@ 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,
>> + * sysrel, and sysretq, which are not core serializing.
>> + */
>> +static inline void sync_core_before_usermode(void)
>> +{
> 
> /* With PTI, we unconditionally serialize before running user code. */
> if (static_cpu_has(X86_FEATURE_PTI))
>  return;

One issue I'm facing with this change is header dependency:
sync_core_before_usermode() is currently implemented in
arch/x86/include/asm/processor.h, but arch/x86/include/asm/cpufeature.h
is needed for static_cpu_has, and it happens to include
asm/processor.h.

I'm facing a similar issue for adding a (in_irq() || in_nmi()) check.

Should we move sync_core_before_usermode() to a different header, and if
so, any suggestion ?

Thanks,

Mathieu



> 
>> +       sync_core();
>> +}
> 
> --Andy

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