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

[Amos Kong]

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.