Re: [Qemu-devel] [PATCH 1/2] avoid to allcate used macaddr to to new nic

[Michael S. Tsirkin]

On Tue, Jun 18, 2013 at 10:05:58AM +0800, Fam Zheng wrote:
> On Mon, 06/17 17:00, Michael S. Tsirkin wrote:
> > On Mon, Jun 17, 2013 at 09:35:10PM +0800, Amos Kong wrote:
> > > QEMU allocates macaddr to nic if user doesn't assigne macaddr.
> > > But we didn't check if the allocated macaddr is used, it might
> > > cause macaddr repeated.
> > > 
> > >  # qemu -device e1000,netdev=h1,mac=52:54:00:12:34:56
> > >   (qemu) device_add e1000
> > >   (qemu) info network
> > >   e1000.0: index=0,type=nic,model=e1000,macaddr=52:54:00:12:34:56
> > >    \ h1: index=0,type=user,net=10.0.2.0,restrict=off
> > >   e1000.1: index=0,type=nic,model=e1000,macaddr=52:54:00:12:34:56
> > > 
> > > This patch adds a check in allocating macaddr, reallocate macaddr
> > > if it's used by other nic.
> > > 
> > > Signed-off-by: Amos Kong <address@hidden>
> > 
> > I'm not sure this is not exactly what was intended in this case.
> > Also this ptotects against an unlikely case of mixing
> > implicit and explicit addresses, but not against
> > a likely case of multiple qemu on same LAN using same MAC.
> 
> IMHO, either way we can do little to protect against collision of
> multiple qemu on the same LAN, but at least this patch protects against
> repeated MAC address in one qemu instance. Better in some degree.

This is a policy, we should not dictate it.
Maybe you want same MAC for some reason?

> Leaving it to user, and asking for address explictly, absolutely helps,
> but makes the interface a bit harder to use: there are still cases user
> wants it generated automatically.

A user that does not want to know what "MAC" even means
is the only one I'm aware of. This is not such a case.

> Just wondering if a random one could be better?

If we are talking about a guest with multiple NICs,
if you generate MACs randomly guest won't know which is which.

It also breaks assumptions guests make that MAC
is a static property of hardware. E.g. it can force windows
re-activation, break resume from suspend etc

> -- 
> Fam