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

[Andy Lutomirski]

On Wed, Jan 17, 2018 at 10:10 AM, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> ----- 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 ?

tlbflush.h, maybe?

>
> Thanks,
>
> Mathieu
>
>
>
>>
>>> +       sync_core();
>>> +}
>>
>> --Andy
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com