On Mon, Sep 21, 2015 at 09:26:11AM -0700, Jesse Barnes wrote:
> On 09/18/2015 12:26 PM, Ville Syrjälä wrote:
> > On Fri, Sep 18, 2015 at 07:44:34PM +0100, Chris Wilson wrote:
> > Yeah I guess that was a crappy example. Trying to think of a better one,
> > I figure execlist status could be read from memory, but apparently
> > that's just mmio. I guess the context update + ELSP write is something
> > we do from the irq handler, but there are plenty of barriers between
> > those two it seems. Maybe there's no good example here.
> >
> >>
> >> Anyway, I thought we had strongly ordered reads on x64/x32?
> >
> > The cpu won't reoder reads vs. reads, or writes vs. writes for that
> > matter if you ignore the nt stuff and whatnot. But AFAIU the whole
> > point of _relaxed() on x86 is that it allows the compiler to reoder
> > memory vs. mmio any which way it wants.
>
> If you mean the top level readX_relaxed() functions, those were more
> about re-ordering on a large I/O fabric than compiler re-ordering. I'm
> not sure how the compiler handles things these days; I thought maybe the
> volatile accesses would imply a re-order barrier as well, since they
> could have side effects the compiler can't anticipate (i.e. re-ordering
> even read after read could change behavior). But it would be good to
> double check... and according to memory-barriers.txt we should be safe
> there, from the readX_relaxed() section:
>
> "Note that relaxed accesses to the same peripheral are guaranteed to be
> ordered with respect to each other."
We have
#define barrier() asm volatile("" ::: "memory")
so I take it someone definitely thinks just volatile isn't enough to
keep the compiler in check.
--
Ville Syrjälä
Intel OTC
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