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

[Rusty Russell <rusty@rustcorp.com.au>]

"Michael S. Tsirkin" <mst@redhat.com> 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 <akong@redhat.com>
>
> 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?

Cheers,
Rusty.