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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

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 ?

I'm not too worried as long as we warn and account them.

Cheers,
Ben.