Re: [Qemu-devel] [RFC qemu PATCH] only writing out the last byte of MAC makes it have effect

[Amos Kong]

On Mon, Mar 25, 2013 at 08:58:49AM +0200, Michael S. Tsirkin wrote:
> On Mon, Mar 25, 2013 at 10:23:57AM +0800, Amos Kong wrote:
> > On Fri, Mar 22, 2013 at 10:45:09AM +1030, Rusty Russell wrote:
> > > "Michael S. Tsirkin" <address@hidden> writes:
> > > > On Thu, Mar 21, 2013 at 02:44:50PM +0800, Amos Kong wrote:
> > > >> The lengcy guests don't have mac programming command, we don't know 
> > > >> when
> > > >> it's safe to use MAC. This patch changed qemu to makes MAC change 
> > > >> effect
> > > >> when the last byte of MAC is written to config space.
> > > >> 
> > > >> MAC address takes first 6 bytes of config space of virtio-net, the addr
> > > >> is 5 when the last byte is written in virtio_config_writeb().
> > > >> 
> > > >> MAC change will effect when n->mac is updated in 
> > > >> virtio_net_set_config().
> > > >> 
> > > >> Signed-off-by: Amos Kong <address@hidden>
> > > >
> > > > Let's see what Rusty says about the spec change.
> > > 
> > > Implementation notes like this belong as a footnote, eg:
> > > 
> > >         For older systems, it is recommended and typical that the device
> > >         write byte 5 of the mac address last, so devices can use that as
> > >         a trigger to commit the mac address change.
> > > 
> > > Now, is this a real, or theoretical issue?  Have we seen this problem in
> > > practice, or should we continue to ignore it?
> > 
> > Hi Rusty, Michael
> > 
> > I didn't touch any problem. MST, and you?
> > 
> > In Linux guest, we should disable the interface before changing mac address.
> >  ifconfig eth0 down
> >  ifconfig eth0 hw ether 10:12:13:14:15:16
> >  ifconfig eth0 up
> >  
> > In Windows 7 guest, after changing mac address in register table,
> > re-enabling interface to make it effect.
> >  reg add HKLM 
> > SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}\0001]
> >  /v NetworkAddress /d 0123456789AB
> >  netsh interface set interface "Local Area Connection" DISABLED
> >  netsh interface set interface "Local Area Connection" ENABLED
> > 
> > 
> > So when we change the mac address, guest os always disable interface
> > to receive all packages. It seems the theoretical issue doesn't exist?
> > 
> > > Cheers,
> > > Rusty.
> > > 
> > 
> > -- 
> >                     Amos.
> 
> Nope. Looks like no spec change is necessary. We already say it's
> not atomic and it looks like guests expect exactly that
> and disable link to prevent strange issues.

I just found Jiri has a commit to allow changing mac when iface
is running. It seems only the virt nics are allowned, right?

But the leagcy guests don't allow to change mac when interface is
running, so no spec change is necessary.

[upstream kernel]
commit d4fc6918f4b3cc844185f59fc518351525950449
Author: Jiri Pirko <address@hidden>
Date:   Wed Jun 27 05:27:46 2012 +0000

    virtio_net: allow to change mac when iface is running
    
    Signed-off-by: Jiri Pirko <address@hidden>
    Signed-off-by: David S. Miller <address@hidden>

RH-BZ: https://bugzilla.redhat.com/show_bug.cgi?id=882868

-- 
                        Amos.