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

[James Yang]

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.)