From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:53012)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1XD63U-0006uf-1E
	for qemu-devel@nongnu.org; Fri, 01 Aug 2014 02:11:13 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1XD63L-0005Ti-0C
	for qemu-devel@nongnu.org; Fri, 01 Aug 2014 02:11:03 -0400
Received: from mail-qa0-x22d.google.com ([2607:f8b0:400d:c00::22d]:59894)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1XD63K-0005Te-RX
	for qemu-devel@nongnu.org; Fri, 01 Aug 2014 02:10:54 -0400
Received: by mail-qa0-f45.google.com with SMTP id cm18so3456160qab.32
	for <qemu-devel@nongnu.org>; Thu, 31 Jul 2014 23:10:53 -0700 (PDT)
Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <53DB2F66.9050205@redhat.com>
Date: Fri, 01 Aug 2014 08:10:46 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1406720388-18671-1-git-send-email-ming.lei@canonical.com>	<1406720388-18671-15-git-send-email-ming.lei@canonical.com>	<53D8FAD7.1070509@redhat.com>
	<20140730151232.GC26313@redhat.com>	<53D90E4C.3090105@redhat.com>	<CACVXFVMgLDUa8aMtcGP4=hDUi__gUn2=vs1a=PEbx=w3Gs9XyA@mail.gmail.com>	<53DA03BF.7030809@redhat.com>
	<CACVXFVNzKTkV4KsSH50D6HtFPZoPdD5t8ktfsFj85YXgBfg=iQ@mail.gmail.com>
In-Reply-To: <CACVXFVNzKTkV4KsSH50D6HtFPZoPdD5t8ktfsFj85YXgBfg=iQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 14/15] hw/block/virtio-blk: create
 num_queues vqs if dataplane is enabled
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/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: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Ming Lei <ming.lei@canonical.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Maydell <peter.maydell@linaro.org>, Fam Zheng <famz@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

Il 01/08/2014 05:09, Ming Lei ha scritto:
>>> >> Per virtio spec, only the feature is set, the VM can be allowed to
>>> >> access the 'num_queues' field, and I didn't see any problem from
>>> >> VM's view point.
>>> >>
>>> >> So could you explain why both dataplane and non-dataplane have
>>> >> to support the feature.
>> >
>> > Because otherwise you change the guest ABI.
> Sorry, I don't understand why the guest ABI is changed since
> VM only parses 'num_queues' iff the feature is set, and the DATAPLANE
> macro is determined during compiling.

Even recompiling the same version of QEMU should not change the guest
ABI.  Recompilation may affect which devices are present, but then the
migration destination will not even start if something is broken.

And as you pointed out, migration from dataplane to non-dataplane will
break because you didn't convert callers of virtqueue_pop/push.

Paolo