Re: VDPA MAC address problem

[Konstantin Shkolnyy <kshk@linux.ibm.com>]

On 3/20/2025 01:54, Eugenio Perez Martin wrote:
> On Thu, Mar 20, 2025 at 1:59 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> Adding Cindy and Eugenio
>>
>> On Thu, Mar 20, 2025 at 12:34 AM Konstantin Shkolnyy <kshk@linux.ibm.com> wrote:
>>>
>>> I’m observing a problem while testing VDPA with Nvidia ConnectX-6 (mlx5)
>>> on s390.
>>>
>>> Upon start, virtio_net_device_realize() tries to set a new MAC address
>>> by VHOST_VDPA_SET_CONFIG which doesn’t do anything.
>>>
>>> Later, the VM gets started and learns about the old address from
>>> virtio_net_get_config() which returns whatever VHOST_VDPA_GET_CONFIG
>>> returns, unless it's "6 zero bytes", in which case it instead returns
>>> the desired new address (and the problem is avoided).
>>>
>>> Then QEMU again tries to set the new address from vhost_net_start(), now
>>> by calling vhost_vdpa_net_load_cmd(...,VIRTIO_NET_CTRL_MAC,
>>> VIRTIO_NET_CTRL_MAC_ADDR_SET, ...). This time the new address is
>>> successfully programmed into the NIC, but the VM doesn't know about it.
>>
>> Have you enabled shadow virtqueue? If yes, does it work if you don't do that?
>>
> 
> Either you're using SVQ or not, is cmdline nic mac address the same as
> the provided with the vdpa command?

The problem happens when the cmdline address (new) is different from the 
current one in the VDPA device (old), whether the old one has been set 
by "vdpa dev add" or by an earlier VM run.

In other words, it's generally not possible to run a VM with an 
arbitrary address - it'll have to inherit whatever the VDPA device 
already has. (Except for the very first run and _only_ if "vdpa dev add" 
didn't specify the address (leaving it "6 zero bytes"), or specified the 
same one as the cmdline.)

Therefore, it's not possible to omit the address in the libvirt XML and 
let libvirt generate it.