On Oct 24, 2013, at 4:45 AM, Benjamin Herrenschmidt wrote:
> On Wed, 2013-10-23 at 23:06 -0500, Kumar Gala wrote:
>> On Oct 23, 2013, at 5:15 AM, Scott Wood wrote:
>>
>>> On Wed, 2013-10-23 at 00:07 -0500, Kumar Gala wrote:
>>>> On Oct 18, 2013, at 2:38 AM, Wolfgang Denk wrote:
>>>>> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
>>>>> index f783c93..f330374 100644
>>>>> --- a/arch/powerpc/kernel/traps.c
>>>>> +++ b/arch/powerpc/kernel/traps.c
>>>>> @@ -986,6 +986,13 @@ static int emulate_instruction(struct pt_regs *regs)
>>>>> return 0;
>>>>> }
>>>>>
>>>>> + /* Emulating the lwsync insn as a sync insn */
>>>>> + if (instword == PPC_INST_LWSYNC) {
>>>>> + PPC_WARN_EMULATED(lwsync, regs);
>>>>> + asm volatile("sync" : : : "memory");
>>>>
>>>> Do we really need the inline asm? Doesn't the fact of just taking an
>>>> exception and returning from it equate to a sync.
>>>
>>> No, it doesn't equate to a sync. See the discussion here:
>>> http://patchwork.ozlabs.org/patch/256747/
>>>
>>
>> Thanks.
>>
>> I'm not sure I'm a fan of doing this as it silently hides a significant
>> performance impact.
>>
>> Could we possible re-write the userspace instruction to be a 'sync' when we
>> hit this?
>
> Rewriting user space is a can of worms I wouldn't get into ... is any
> other arch doing it ?
Fair enough
>
> I'm not too worried as long as we warn and account them.
Than, I'd ask this be under a Kconfig option that is disabled by default.
Users should have to explicitly enable this so they know what they are doing.
- k
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