On Thu, 9 Mar 2006, Paul Mackerras wrote:
... and x86 mmiowb is a no-op. It's not x86 that I think is buggy.
x86 mmiowb would have to be a real op too if there were any multi-pathed PCI buses out there for x86, methinks.
Basically, the issue boils down to one thing: no "normal" barrier will _ever_ show up on the bus on x86 (ie ia64, afaik). That, together with any situation where there are multiple paths to one physical device means that mmiowb() _has_ to be a special op, and no spinlocks etc will _ever_ do the serialization you look for.
Put another way: the only way to avoid mmiowb() being special is either one of:
(a) have the bus fabric itself be synchronizing
(b) pay a huge expense on the much more critical _regular_ barriers
Now, I claim that (b) is just broken. I'd rather take the hit when I need to, than every time.