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

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

----- On Jan 17, 2018, at 1:13 PM, Andy Lutomirski luto@kernel.org wrote:

> On Wed, Jan 17, 2018 at 10:10 AM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>> ----- On Jan 17, 2018, at 12:53 PM, Andy Lutomirski luto@kernel.org wrote:
>>
>>> On Wed, Jan 17, 2018 at 8:54 AM, Mathieu Desnoyers
>>> <mathieu.desnoyers@efficios.com> 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@efficios.com>
>>>> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
>>>> 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: 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
>>>> ---
>>>> 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?

Core serialization seems to be unrelated to TLB flushing though.

I'm considering to create those new header files:

include/asm-generic/sync_core.h (empty function)
arch/x86/include/asm/sync_core.h

for the sync_core_before_usermode() static inlines. Any better idea ?

Thanks,

Mathieu


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

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