On Oct 24, 2013, at 4:05 PM, James Yang wrote:
> On Thu, 24 Oct 2013, Kumar Gala wrote:
>
>> 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.
>
>
> I think it should be enabled by default, rather than disabled, so that
> users would actually see a warning rather than get a sig 4. Or, let
> it not be Kconfig-able so that this doesn't become a problem any more.
> (It's been 4 years since I sent to you an earlier version of this
> patch.)
And clearly most users don't seem terrible annoyed enough about this issue to
have concerns. I don't see why making it a Kconfig option impacts the small
handful of people that happen to try and run a more generic distro on e500
cores.
- k
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