Re: lfence?

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Wed, Mar 15, 2006 at 05:06:03PM -0500, Thor Lancelot Simon wrote:
> On Wed, Mar 15, 2006 at 04:51:55PM -0500, Jed Davis wrote:
> > 
> > As for sfence: it's part of SSE, and according to Intel needed only if
> > memory is explicitly mapped as write-combining or accessed with a
> > special "non-temporal" store insn.  According to comments in Linux,
> > there exist non-Intel clones that do out-of-order store, and a
> > non-noop sfence is needed there, but they don't enable it by default.
> > It's lfence and mfence that are part of SSE2.
> 
> Do we need to worry about BIOSes leaving video or other device memory
> mapped as write-combining?

It would be a problem, yes, but not only for Xen. We should have a
MI interface for these *fence functions; some drivers should in theory need
them (e.g. esiop(4), which implement some kind of lock-free data structure
between the host and adapter's processor using DMA memory).

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--