Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

[Trent Piepho]

On Fri, 23 May 2008, Benjamin Herrenschmidt wrote:
> On Tue, 2008-05-20 at 15:53 -0700, David Miller wrote:
> > From: Scott Wood <scottwood@xxxxxxxxxxxxx>
> > Date: Tue, 20 May 2008 17:43:58 -0500
> > > David Miller wrote:
> > > > The __volatile__ in the asm construct disallows movement of the
> > > > inline asm relative to statements surrounding it.
> > > >
> > > > The only reason barrier() in kernel.h needs a memory clobber is
> > > > because of a bug in ancient versions of gcc.  In fact, I think
> > > > that memory clobber might even be removable.
> > >
> > > Current versions of GCC seem quite happy to move non-asm memory accesses
> > > around a volatile asm without a memory clobber; see the test Trent posted.
> >
> > Indeed, and even the GCC manual is clear about this.
>
> So what is the scope of that problem ?
>
> IE. Take an x86 version of that test, writing to memory, doing a writel
> to some MMIO, then another memory write, can those be re-ordered with
> the current x86 version of writel ?

Yes, the same thing can happen on x86.  As far as I could tell, this is
something that all other arches can have happen.  Usually aliasing prevents
it, but it's not hard to constuct a test case where it doesn't.
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