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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 17, 2015 at 03:52:24PM +0000, Will Deacon wrote:
> On Thu, Dec 17, 2015 at 04:09:17PM +0100, Peter Zijlstra wrote:
> > On Thu, Dec 17, 2015 at 04:34:57PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Dec 17, 2015 at 03:02:12PM +0100, Peter Zijlstra wrote:
> > 
> > > > > 	commit 9e1a27ea42691429e31f158cce6fc61bc79bb2e9
> > > > > 	Author: Alexander Duyck <alexander.h.duyck@redhat.com>
> > > > > 	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.
> > > 
> > > Does outer coherent imply inner coherent?
> > > 
> > > > the *mb() primitives cover both.
> > 
> > I do not think so, but lets add Will, he dreams this stuff.
> 
> Right, and I don't sleep well these days.
> 
> Anyway, the outer-shareable domain (osh) is a superset of the
> inner-shareable domain (ish). The inner-shareable domain contains the
> CPUs and any peripherals that you and I would call "cache coherent". The
> outer-shareable domain extends this to cover a strange set of "less cache
> coherent" devices, which we would just call "not cache coherent" for the
> purposes of Linux. Normal, non-cacheable memory (i.e. the memory type we
> use for non-coherent DMA buffers) is outer-shareable.
> 
> Since the barrier macros don't know if the device is coherent or not, we
> use the stronger semantics of outer-shareable.
> 
> I've not been following the thread, but I reckon we could add dma_mb()
> (as dmb(osh) on arm), then use that to build dma_load_acquire and
> dma_store_release accessors. On arm64, we could probably use the
> acquire/release instructions directly, since they inherit the shareability
> domain of the address (which has the nice property of being inner-shareable
> for coherent devices).
> 
> The massive pain with adding new accessors is defining the semantics.
> memory-barriers.txt is already lacking for the CPU side, and we're
> struggling to express the kind of transitivity guarantees we provide
> today, let alone with new primitives :(
> 
> Will

Well virtio (might) have wanted dma_mb in the past.

But if are adding barrier stuff anyway, we really want
pv_ counterparts to smp_ that do the same on CONFIG_SMP
but don't change when CONFIG_SMP is disabled.

-- 
MST