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

[Kumar Gala]

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