[ltt-dev] [URCU PATCH] cmm: provide lightweight rmb/wmb on PPC

[Paolo Bonzini]

On 09/20/2011 06:51 PM, Mathieu Desnoyers wrote:
>> >  I think you're right. "eieio;lwsync" is good for rmb, lwsync is good for
>> >  wmb/smp_rmb/smp_wmb.
> I'm not convinced that the "eieio; lwsync" combo would provide the
> ordering we're looking for for cmm_rmb(). AFAIK, eieio orders,
> separately, a) cacheable stores and b) loads and stores to non-cacheable
> memory. AFAIK, lwsync orders cacheable memory ops, but not loads with
> respect to previous stores. So basically, this combo lacks ordering of
> non-cacheable memory accesses with respect to cachable memory accesses.

Yeah, better safe than sorry.

> Why would lwsync be good for cmm_wmb ? Does it order non-cacheable
> writes ?

The manuals say non-cacheable writes are always ordered.

Paolo