Re: [rfc] add io barriers, remove mmiowb

[Jes Sorensen <jes@sgi.com>]

>>>>> "Nick" == Nick Piggin <npiggin@suse.de> writes:

Nick> mb and wmb are now no longer guaranteed to order system memory
Nick> operations with device memory stores. mmiowb has been introduced
Nick> to provide this ordering (when combined with a mb, wmb, or
Nick> spin_unlock). Unfortunately, it appears to be rather less well
Nick> understood among both users and implementors than even the old
Nick> memory barrier scheme. It also subtly breaks existing code that
Nick> uses mb or wmb (if only on sn2). I really think it is not a good
Nick> solution.

Nick> The alternative I propose is to restore mb and wmb to their full
Nick> strength.  This does mean that sn2 has to do the equivalent of
Nick> mb+mmiowb, wmb+mmiowb respectively, but that's the price you pay
Nick> for weak memory ordering.

Nick,

Introducing this constraint would make me less than pleased I have to
admit. It's a very expensive operation to do since it requires going
out talking to the PCI bridge, doing that on every wmb() is going to
really hurt :-(

Cheers,
Jes