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Date: Thu, 17 Jan 2013 09:39:54 +0100
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20130117083954.GA19648@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [QEMU PATCH v2] virtio-net: introduce a new
 macaddr control
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To: Amos Kong <akong@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>, virtualization@lists.linux-foundation.org, qemu-devel@nongnu.org, kvm@vger.kernel.org, mst@redhat.com

On Thu, Jan 17, 2013 at 01:45:11PM +0800, Amos Kong wrote:
> On Thu, Jan 17, 2013 at 11:49:20AM +1030, Rusty Russell wrote:
> > akong@redhat.com writes:
> > > @@ -349,6 +351,14 @@ static int virtio_net_handle_mac(VirtIONet *n, uint8_t cmd,
> > >  {
> > >      struct virtio_net_ctrl_mac mac_data;
> > >  
> > > +    if (cmd == VIRTIO_NET_CTRL_MAC_ADDR_SET && elem->out_num == 2 &&
> > > +        elem->out_sg[1].iov_len == ETH_ALEN) {
> > > +        /* Set MAC address */
> > > +        memcpy(n->mac, elem->out_sg[1].iov_base, elem->out_sg[1].iov_len);
> > > +        qemu_format_nic_info_str(&n->nic->nc, n->mac);
> > > +        return VIRTIO_NET_OK;
> > > +    }
> > 
> > Does the rest of the net device still rely on the layout of descriptors?
> 
> No, only info string of net client relies on n->mac

I think the question is whether the hw/virtio-net.c code makes
assumptions about virtqueue descriptor layout (e.g. sg[0] is the header,
sg[1] is the data buffer).

The answer is yes, the control virtqueue function directly accesses
iov[n].

Additional patches would be required to convert the existing
hw/virtio-net.c code to make no assumptions about virtqueue descriptor
layout.  It's outside the scope of this series.

Stefan