qemu network adapter initialization fails when using macaddr=<multicast MAC-address>

[Roman Fiedler]

Not sure if ultra-strange, nondocumented feature in qemu (or linux kernel) or really bug: Network card initialization fails if first byte of mac address is not 00. The problem occurs at least with model=pcnet/rtl8139, in both cases the network adapter is not usable.

How to reproduce:

* Take standard initrd/kernel (tested with hardy)

* Start qemu (cmd see below) and enter "modprobe pcnet32" at prompt:
qemu -name SetupTest -no-acpi -m 128 -drive file=/dev/null,if=ide,index=0 -net nic,macaddr=00:22:33:44:55:66,model=pcnet -net user -kernel vmlinuz-2.6.24-26-generic -initrd initrd.img-2.6.24-26-generic -append break=premount

You will see "pcnet32 ... at 0x..., 00:22:33:44:55:66

* Do same with mac address 11:22:33:44:55:66
qemu -name SetupTest -no-acpi -m 128 -drive file=/dev/null,if=ide,index=0 -net nic,macaddr=11:22:33:44:55:66,model=pcnet -net user -kernel vmlinuz-2.6.24-26-generic -initrd initrd.img-2.6.24-26-generic -append break=premount

You will see "pcnet32 ... at 0x..., 00:00:00:00:00:00

The network adapter is non-functional, "ip link set eth0 up" does not report error, but does not work (indicates at least some linux kernel influence)

With the rtl8139 adapter, mac-address in guest is correct, but adapter does not work either (indicates qemu influence)

[Roman Fiedler (roman-fiedler-deactivatedaccount)]

Tested other mac addrs, seems that the first byte has to be even. Would it make sense to issue a warning if a user requests a mac address with an odd first byte? What is the special meaning of the bit 0?

[Anthony Liguori (anthony-codemonkey)]

That bit indicates whether the address is a multicast or unicast address. A network card cannot have a multicast address.

It would make sense to do sanity checking for this.

[Thomas Huth (th-huth)]

A check for multicast MAC addresses has been introduced by this commit here:
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=d60b20cf2ae6644b051
So I think we can close this ticket now.