Re: [PATCH] fixes for the SLB shadow buffer

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

On Thu, 2007-08-02 at 15:56 +1000, Michael Neuling wrote:
> 
> But even in the case of a checkpoint restart, the ordering will be
> preserved as the NIA we get as part of the checkpoint will have all
> previous instructions complete and none of the following instructions
> started.

Instruction completion isn't enough to ensure storage ordering. The
stores may well be complete but the data still in separate store queues.

> So I guess the questions is, does PHYP even need to access the shadow
> buffer of another CPU, while that other CPU is in flight.  I'm not
> sure
> that they can as they can't read the entire buffer atomically if the
> target CPU is still active.  So PHYP must stop instructions on the
> target CPU, before it reads it's shadow buffer.  Hence no ordering
> problems.
> 
> I should probably talk to some PHYP guys to confirm, but i think we
> can
> remove all the barriers when writing to the shadow buffer

Bah, just keep then in, eieio's won't hurt much and it doesn't look like
a critically hot code path.

Ben.