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Date: Fri, 14 Sep 2012 17:01:43 +0800
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Subject: Re: [Qemu-devel] [PATCH v2] rtl8139: implement 8139cp link status
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: aliguori@us.ibm.com, Amos Kong <akong@redhat.com>, mst@redhat.com, qemu-devel@nongnu.org, stefanha@linux.vnet.ibm.com

On 09/14/2012 03:30 PM, Stefan Hajnoczi wrote:
> On Fri, Sep 14, 2012 at 2:34 AM, Amos Kong<akong@redhat.com>  wrote:
>> On 13/09/12 20:29, Stefan Hajnoczi wrote:
>>> On Thu, Sep 13, 2012 at 9:51 AM, Amos Kong<akong@redhat.com>  wrote:
>>> Regarding migration: do we migrate the NetClient->link_down field? If
>>> we only migrate the status register value then the link may actually
>>> be up at the net.c level.
>>
>> I tried to add 'MediaStatus' to 'struct RTL8139State', and update
>> 'VMStateDescription vmstate_rtl8139', then the value of MediaStatus
>> will be migrated.
>>
>> But the idea in v2 is better.
> Migrating the NIC's media status is not enough.  Above I asked about
> migrating nc->link_down, which determines whether net.c delivers
> packets or drops them.
>
> Your patch migrates the NIC's media status but I believe nc->link_down
> isn't being migrated and the guest will therefore receive packets from
> the host!  This could lead to unexpected results since the guest
> thinks the link is down.
>
> It's not a bug in your patch, but a larger issue that needs to be
> addressed for all NICs that support migration.  (Unless I missed the
> code that will migrate link_down.)
>
> Stefan

A possible solution is to infer the nc->link_down from the media status 
register in the destination. It works without adding more codes net.c 
but need model specific callback functions.