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SubjectRe: Memory Barrier Definitions
justincarlson@cmu.edu wrote:
>
> On Tue, 2002-05-07 at 16:27, Alan Cox wrote:
> > and our current heirarchy is a little bit more squashed than that. I'd
> > agree. We actually hit a corner case of this on the IDT winchip x86 where
> > we run relaxed store ordering and have to define wmb() as a locked add of
> > zero to the top of stack - which does have a penalty that isnt needed
> > for CPU ordering.
> >
> > How much of this impacts Mips64 ?
>
> In terms of the MIPS{32|64} ISA, the current primitives seem fine;
> there's only 1 option defined in the ISA: 'sync'. Order for all
> off-cache accesses is guaranteed around a sync.
>
> It gets a bit more complicated when you talk about what particular
> implementations do, and ordering rules for uncached vs cached accesses,
> but to the best of my knowledge there aren't any fundamental problems as
> described for the PPC.
>
> -Justin

PPC also guarantees every ordering when using the 'sync' instruction, so
that will give correctness at the price of a 1000 cycles or so. You
refer to different rules for cached vs uncached on other implementations
-- that is the essence of our problem. Are there different barrier
instructions in MIPS which provide different levels of performance for
different ordering enforcements?

Dave.
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