From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=44623 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1OKUtU-0003nO-Vb
	for qemu-devel@nongnu.org; Fri, 04 Jun 2010 07:16:58 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <rusty@rustcorp.com.au>) id 1OKUtU-00013t-1o
	for qemu-devel@nongnu.org; Fri, 04 Jun 2010 07:16:56 -0400
Received: from ozlabs.org ([203.10.76.45]:40260)
	by eggs.gnu.org with esmtp (Exim 4.69)
	(envelope-from <rusty@rustcorp.com.au>) id 1OKUtT-00013W-NL
	for qemu-devel@nongnu.org; Fri, 04 Jun 2010 07:16:56 -0400
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Fri, 4 Jun 2010 20:46:49 +0930
References: <cover.1275403477.git.mst@redhat.com>
	<201006041204.57973.rusty@rustcorp.com.au>
	<20100604103543.GA22270@redhat.com>
In-Reply-To: <20100604103543.GA22270@redhat.com>
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Message-Id: <201006042046.49872.rusty@rustcorp.com.au>
Subject: [Qemu-devel] Re: [PATCHv3 1/2] virtio: support layout with avail
	ring before idx
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org

On Fri, 4 Jun 2010 08:05:43 pm Michael S. Tsirkin wrote:
> On Fri, Jun 04, 2010 at 12:04:57PM +0930, Rusty Russell wrote:
> > On Wed, 2 Jun 2010 12:17:12 am Michael S. Tsirkin wrote:
> > > This adds an (unused) option to put available ring before control (avail
> > > index, flags), and adds padding between index and flags. This avoids
> > > cache line sharing between control and ring, and also makes it possible
> > > to extend avail control without incurring extra cache misses.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> > No no no no.  254?  You're trying to Morton me![1]
> 
> Hmm, I wonder what will we do if we want a 3rd field on
> a separate chacheline. But ok.
> 
> > How's this (untested):
> 
> I think we also want to put flags there as well,
> they are used on interrupt path, together with last used index.

I'm uncomfortable with moving a field.

We haven't done that before and I wonder what will break with old code.

Should we instead just abandon the flags field and use last_used only?
Or, more radically, put flags == last_used when the feature is on?

Thoughts?
Rusty.