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

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Message-ID: <20151217140212.GB6344@twins.programming.kicks-ass.net>
Date:	Thu, 17 Dec 2015 15:02:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Jason Wang <jasowang@...hat.com>, qemu-devel@...gnu.org
Subject: Re: [PATCH] virtio_ring: use smp_store_mb

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.

the *mb() primitives cover both.
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