Re: [PATCH] virtio_ring: use smp_store_mb

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	Jason Wang <jasowang@redhat.com>,
	qemu-devel@nongnu.org,
	Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [PATCH] virtio_ring: use smp_store_mb
Date: Thu, 17 Dec 2015 16:34:57 +0200	[thread overview]
Message-ID: <20151217163412-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <20151217140212.GB6344@twins.programming.kicks-ass.net>

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@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.