linux-kernel - Re: [PATCH] virtio_ring: use smp_store

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Message-ID: <20151217163412-mutt-send-email-mst@redhat.com>
Date:	Thu, 17 Dec 2015 16:34:57 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Jason Wang <jasowang@...hat.com>, qemu-devel@...gnu.org,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH] virtio_ring: use smp_store_mb

On Thu, Dec 17, 2015 at 03:02:12PM +0100, Peter Zijlstra wrote:
> 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 <alexander.h.duyck@...hat.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.
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