From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:35078)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <mst@redhat.com>) id 1V2KnS-0006Rq-QE
	for qemu-devel@nongnu.org; Thu, 25 Jul 2013 08:37:34 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <mst@redhat.com>) id 1V2KnN-0003UP-TZ
	for qemu-devel@nongnu.org; Thu, 25 Jul 2013 08:37:30 -0400
Received: from mx1.redhat.com ([209.132.183.28]:60083)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <mst@redhat.com>) id 1V2KnN-0003UE-Ix
	for qemu-devel@nongnu.org; Thu, 25 Jul 2013 08:37:25 -0400
Date: Thu, 25 Jul 2013 15:38:30 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
Message-ID: <20130725123830.GA403@redhat.com>
References: <1373661422-23606-1-git-send-email-peter.maydell@linaro.org>
	<1373661422-23606-4-git-send-email-peter.maydell@linaro.org>
	<20130725053841.GA15500@redhat.com>
	<CAFEAcA8TEdkGpCXCTCyVkN7TiaWA9WGA2FM3OtQyQsXAkJF-_A@mail.gmail.com>
	<20130725090317.GA26648@redhat.com>
	<CAFEAcA8d9bD9eK7f7fbNeiAjBpePeBM=bjxvMxsdD4=WjD1D-g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAFEAcA8d9bD9eK7f7fbNeiAjBpePeBM=bjxvMxsdD4=WjD1D-g@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH v2 3/8] virtio: Add support for guest
 setting of queue size
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: Peter Maydell <peter.maydell@linaro.org>
Cc: Anthony Liguori <aliguori@us.ibm.com>, patches@linaro.org, Jason Wang <jasowang@redhat.com>, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org, kvmarm@lists.cs.columbia.edu, David Gibson <david@gibson.dropbear.id.au>

On Thu, Jul 25, 2013 at 01:30:15PM +0100, Peter Maydell wrote:
> On 25 July 2013 10:03, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Thu, Jul 25, 2013 at 09:50:21AM +0100, Peter Maydell wrote:
> >> On 25 July 2013 06:38, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > Probably needs to go back to default value on reset?
> >>
> >> Tricky, since the default value is "whatever was passed to
> >> virtio_add_queue()" and we don't save that anywhere.
> >>
> >> For virtio-mmio it is a guest bug to fail to write to the
> >> QueueNum register, so the current behaviour is not out of
> >> specification (and not harmful either AFAICT).
> >
> > Best not to leak info across reboots.
> > Also if guest sets num = 0 it will cause all kind of
> > harm, no?
> >
> >> I guess we could add a vring.defaultnum, which would be
> >> set by virtio_add_queue/virtio_del_queue, and have reset
> >> copy defaultnum into num. No migration needed for defaultnum
> >> because it's always the same for a particular qemu config.
> 
> So I had a look at implementing this, and I noticed that
> we already have some odd behaviour on reset. Specifically,
> virtio backends like net can create virtio queues based on
> guest behaviour (ie setting feature bits). But on reset,
> these queues aren't deleted, so a post-reset QEMU will look
> different from a from-scratch restarted QEMU...

That's a bug. Thanks for the report. :)

> This in turn makes 'save defaultnum and have reset copy it
> into num' awkward, because defaultnum now needs to be
> migrated (otherwise it would do the wrong thing on a reset
> after a VM migration).
> 
> -- PMM

Looks like we'll have to fix the bug first :(