Re: [Qemu-devel] [PATCH] virtio_ring: use smp_store_mb

[Peter Zijlstra]

On Thu, Dec 17, 2015 at 03:26:29PM +0200, Michael S. Tsirkin wrote:
> > Note that virtio_mb() is weirdly inconsistent with virtio_[rw]mb() in
> > that they use dma_* ops for weak_barriers, while virtio_mb() uses
> > smp_mb().
> 
> It's a hack really. I think I'll clean it up a bit to
> make it more consistent.
> 
> To simplify things, you may consider things before
> the optimization brought in by
>       commit 9e1a27ea42691429e31f158cce6fc61bc79bb2e9
>       Author: Alexander Duyck <address@hidden>
>       Date:   Mon Apr 13 21:03:49 2015 +0930
> 
>           virtio_ring: Update weak barriers to use dma_wmb/rmb

That commit doesn't make any sense. dma_*mb() explicitly does _NOT_
cover the smp_*mb() part.

Again, look at the ARM definitions, the smp_*mb() primitives use the
inner coherence stuff, while the dma_*mb() primitives use the outer
coherent stuff.

the *mb() primitives cover both.