Internal MAC addresses list (mac_table) usage

[Ovchinnikov, Vitalii]

Hi folks,

While developing an Ethernet NIC model I noticed that QEMU maintains the 
following internal array which marks used/free MAC addresses in net/net.c:

static int mac_table[256] = {0};

with three private (static) functions accessing it: qemu_macaddr_set_used, 
qemu_macaddr_set_free, qemu_macaddr_get_free.
Public (non-static) interface to this array includes two functions: 
qemu_macaddr_default_if_unset and qemu_del_nic.

The vast majority of existing NIC models calls qemu_macaddr_default_if_unset in 
their *_realize functions replacing zeroed-out MAC address with the free one 
returned by QEMU, for instance (lan9118_realize functions from 
hw/net/lan9118.c):

   ...
    qemu_macaddr_default_if_unset(&s->conf.macaddr);

    s->nic = qemu_new_nic(&net_lan9118_info, &s->conf,
                          object_get_typename(OBJECT(dev)), dev->id, s);
    qemu_format_nic_info_str(qemu_get_queue(s->nic), s->conf.macaddr.a);
   ...

qemu_del_nic is being called from net_cleanup function right before QEMU 
finishes execution.

What appears to be a possible SW architecture gap is that NIC models have no 
means to inform QEMU about changing their MAC addresses during execution (again 
from hw/net/lan9118.c, do_mac_write function):

    case MAC_ADDRH:
        s->conf.macaddr.a[4] = val & 0xff;
        s->conf.macaddr.a[5] = (val >> 8) & 0xff;
        lan9118_mac_changed(s);
        break;
    case MAC_ADDRL:
        s->conf.macaddr.a[0] = val & 0xff;
        s->conf.macaddr.a[1] = (val >> 8) & 0xff;
        s->conf.macaddr.a[2] = (val >> 16) & 0xff;
        s->conf.macaddr.a[3] = (val >> 24) & 0xff;
        lan9118_mac_changed(s);
        break;

lan9118_mac_changed function here simply changes NIC info string using 
qemu_format_nic_info_str, hence stale MAC address stays marked as used in the 
mac_table whereas it's not actually in use any more.

Am I right in thinking of it as a SW architecture gap/bug that needs to be 
addressed?

BR,
Vitalii