Re: [Qemu-devel] [PATCH] qemu-bridge-helper: force usage of a very high MAC address for the bridge

[Paolo Bonzini]

Il 22/03/2013 22:37, Corey Bryant ha scritto:
> Is it desirable to change a mac address under the covers?

This is the TAP mac address.  It is unrelated to the guest's MAC
address.  It is a random link-local address, all this patch does is make
it less random.

> Also it seems like this might be better if it was optional.
> qemu-bridge-helper can take command line options like this:
> 
> -net tap,helper="/usr/local/libexec/qemu-bridge-helper --br=br0"
> 
> Perhaps adding a --macaddr option is a better approach?

Adding a --macaddr=00:11:... option to force a _particular_ MAC address
could also be useful (if only to help associating guests' MAC addresses
with tap interfaces; the only difference would be the initial 0xFE
byte).  But it is not easily done in QEMU because MAC addresses are a
-device option, not a -netdev option.

This patch fixes the real issue that, when the bridge changes MAC
address, there is a small but significant network downtime.  See libvirt
commit 6ea90b843eff95be6bcbb49a327656fc6f6445ef for reference.

What would you make it conditional on?

Paolo