Re: wmb vs mmiowb

Jesse Barnes
2007-08-23T16:56:16Z

On Thursday, August 23, 2007 12:27 am Benjamin Herrenschmidt wrote:
> > Of course, the normal memory barrier would usually be a
> > "spin_unlock()" or something like that, not a "wmb()". In fact, I
> > don't think the powerpc implementation (as an example of this) will
> > actually synchronize with anything *but* a spin_unlock().
>
> We are even more sneaky in the sense that we set a per-cpu flag on
> any MMIO write and do the sync automatically in spin_unlock() :-)

Yeah, that's a reasonable thing to do, and in fact I think there's code
to do something similar when a task is switched out (this keeps user
level drivers from having do mmiowb() type things).

FWIW, I think I had an earlier version of the patch that used the name
pioflush() or something similar, the only confusing thing about that
name is that the primitive doesn't actually force I/Os down to the
device level, just to the closest bridge.

It'll be interesting to see if upcoming x86 designs share this problem
(e.g. large HT or CSI topologies).

Jesse