From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:54542)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@redhat.com>) id 1Tiof8-0006BB-MC
	for qemu-devel@nongnu.org; Wed, 12 Dec 2012 10:56:10 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <stefanha@redhat.com>) id 1Tiof1-0004aV-5a
	for qemu-devel@nongnu.org; Wed, 12 Dec 2012 10:55:58 -0500
Received: from mx1.redhat.com ([209.132.183.28]:44588)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@redhat.com>) id 1Tiof0-0004aK-Sd
	for qemu-devel@nongnu.org; Wed, 12 Dec 2012 10:55:51 -0500
Date: Wed, 12 Dec 2012 16:55:47 +0100
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20121212155547.GC16956@stefanha-thinkpad.redhat.com>
References: <1355144985-12897-1-git-send-email-stefanha@redhat.com>
	<1355144985-12897-5-git-send-email-stefanha@redhat.com>
	<20121211141820.GB18753@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20121211141820.GB18753@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v6 04/12] dataplane: add virtqueue vring
	code
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: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, khoa@us.ibm.com, Paolo Bonzini <pbonzini@redhat.com>, Asias He <asias@redhat.com>

On Tue, Dec 11, 2012 at 04:18:20PM +0200, Michael S. Tsirkin wrote:
> On Mon, Dec 10, 2012 at 02:09:37PM +0100, Stefan Hajnoczi wrote:
> > +        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, desc.len,
> > +                                       desc.flags & VRING_DESC_F_WRITE);
> > +        if (!iov->iov_base) {
> > +            error_report("Failed to map indirect descriptor"
> > +                         "addr %#" PRIx64 " len %u",
> > +                         (uint64_t)desc.addr, desc.len);
> > +            vring->broken = true;
> > +            return -EFAULT;
> > +        }
> > +        iov->iov_len = desc.len;
> > +        iov++;
> 
> Hmm, this assumes a descriptor can not cross a memory
> region boundary. Is this really guaranteed?
> vhost does not make such assumptions.

Just responded in the other subthread.

hw/virtio.c doesn't split across memory region boundaries.  This suggest
we don't hit this in practice.  If it does we'll print the error and
disable the device until virtio reset.

I can add it to my TODO list.  hw/virtio.c also uses exit(1) for
guest-triggerable errors at runtime and I'd like it to do something like
vring->broken above.

IMO it's not worth trying to perfect these things here.  The next step
after this series should be looking at thread-safe memory API so that we
can use hw/virtio.c and improve it for all users.

Stefan