[PATCH] [RFC] Emulate "lwsync" to run standard user land on e500 cores

[Kumar Gala]

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