Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: removesingle ipi fallback for smp_call_function_many())

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 
> 
> On Wed, 18 Feb 2009, Ingo Molnar wrote:
> >
> > an off-list comment pointed out this piece of information as 
> > well:
> > 
> >   http://www.sandpile.org/ia32/coherent.htm
> > 
> >   A WRMSR to one of the x2APIC MSRs (0000_0800h...0000_0BFFh) is 
> >   not guaranteed to be serializing.
> > 
> > So i suspect we should just enclose it in smp_mb() pairs to make 
> > sure it's a full barrier in both directions?
> 
> Why would we care about "both directions"?
> 
> I think putting an sfence _before_ the wrmsr (and not even all 
> of them - just put it in front of the "send IPI" sequence) 
> should be fine. Any other ordering sounds like just 
> unnecessary overhead to me.
> 
> We do want this to be low-overhead, even if we probably don't 
> care _that_ much.

yeah, you are right, making sure prior stores become visible 
should be the only worry here.

	Ingo
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